Flood

Were you looking for the novel Halo: The Flood, or The Flood, the ninth level in Halo Wars?

"One single flood spore can destroy a species."

- Rtas 'Vadumee

The Flood, (Latin Inferi redivivus meaning "The dead reincarnated" ), or The Parasite as it's called by the Covenant, are extragalactic inter-connected collective parasitic organisms that survive by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood are responsible for consuming most of the sentient life in the galaxy, including the Forerunners, during the 931-year-long Forerunner-Flood War. The Flood presents the most variable faction in the trilogy, as it can infect and mutate Human and Covenant species, such as Elites and Brutes, into Combat Forms.

Background
The Flood are extremely adaptive, dangerous, and expendable parasitic lifeforms. They were first encountered in the Milky Way Galaxy by a Forerunner pioneer group on planet G617 g1. It is a highly infectious viral supercell, and the cause of the "Forerunner-Flood War", which lasted approximately 931 years. They were the reason that the Halo Array was constructed; as a last final effort by the Forerunners to preserve biological diversity in the galaxy. After exhausting every last strategic option, the Halos were fired, resulting in the elimination of the original Flood infestation.

The Flood exists in a fairly stable state in the absence of a Gravemind. They lack the ability to spawn spontaneously and form around sentient life forms, specifically those that are self-aware and capable of introspection. This stage of a Flood outbreak is known as the Feral Stage.

During its Feral Stage, the Flood is only capable of local coordination through use of pheromone-based communication. Research indicates that the content of said messages are of very limited complexity. Individual Combat and Carrier Forms have access to the skills and memories of their host; however, once an outbreak establishes a viable Gravemind, the Coordinated Stage begins—and it is at this point the Flood becomes truly dangerous.

Synchronization among remote groups of varied forms and the use of occupational specializations by members within local and remote groups can no longer be accounted for by the use of pheromone-based communication. Group reactions to situational changes at a strategic level are instantaneous or near-instantaneous with no perceptible time lag attributable to distance. It has been theorized that post Feral Stage Flood have some form of telepathy, although this has not been scientifically verified.

A Gravemind (like the Proto-Gravemind that gave rise to it) is the fusion of pure Flood Super Cells and the evolved complex neuro-system of one or more sentient entities. The control it exerts over the more mobile forms it creates is accurately likened to an entity's control over its own limbs. Ergo, during the Feral stage, control is localized at the individual level, but once an outbreak enters the Coordinated stage control is assumed by, and broadcast from, a central compound intelligence.

Although no data exists from any adequately controlled experiment, data gathered in the field suggests that if a mobile form is removed from the influence of the Gravemind coordinating its group of origin it will revert to a Feral Stage. Insufficient data exists on whether mobile forms of one Gravemind would be influenced by another Gravemind, though logic dictates this would be the case. However, because individual Graveminds are remarkably similar and are all striving for the same goal, it is likely that a distinction would be impossible to detect.

The Bestiarum states that the society of the Flood closely resembles an "utopian socialist ideal as there is no wealth, no poverty, no want, no crime, and no disease; every individual works for the advancement of the Flood as a whole". It also states that it "is unfortunate that their one overriding goal is to [butcher and consume] every other life form in the galaxy".

Forerunner-Flood War
"The only way to stop the Flood is to starve them to death."

- Cortana to John 117 and 343 Guilty Spark.

The Flood are believed to have arrived in the Milky Way Galaxy more than 100,000 years prior to 2552 when they encountered the Forerunners on the planet G617g. The Flood used unarmed civilian assets to penetrate planetary defense groups, first striking at the Forerunner-held planet of LP 656-38 e, bypassing the Forerunner Orbital Fleet, and infesting the planet below. They soon moved to similarly infest the planet DM-3-1123b. They were perceived as hostile by the Forerunners, whose ground forces were overcome by the sheer numbers of Flood forms on the planets. While at first the Flood demonstrated a lack of even "basic cohesion", they had "numerical superiority"; billions of forms dedicated to the assault, and every member of the Forerunner population a potential Flood host. In desperation, the Forerunner's armada was ordered to immediately commence full planetary bombardment on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before bombardment commenced, leaving the Forerunner military with shallow victories against the Flood. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers appeared to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.

Thereafter, the Forerunners realized that ordinary naval tactics were unable to stem the growing tide of the Flood, and decided to pin their hopes of defeating the Flood on super weaponry. While the Forerunners initially deployed the Sentinel robotic drones to fight against and contain the Flood, almost immediately afterward, the Forerunner Fleet Command contemplated "Premature stellar collapse": using naval battle groups to destroy planetary star systems' primary stars in supernova that would engulf the planetary systems and prevent any possibility of Flood infection.

While the Flood were exponentially growing, spreading from system to system with the intent to infect the Forerunner population, not to wage war with Forerunner battle groups, the tide was turning: the Flood had reached sufficient proportions to create a Gravemind form, a creature that embodied a collective sentient intelligence for the Flood species, and one that could coordinate the Flood swarms in intelligent attacks against the Forerunner Fleets. The Forerunners were aware of this, and constructed the Contender class artificial intelligence Mendicant Bias in an effort to destroy this centralized Gravemind and disorganize the Flood. This too failed since Mendicant Bias went rampant and joined the Flood cause to destroy its former masters.

The Forerunners began a "Conservation Measure", researching the Flood, while constructing a Portal to the Ark on Earth and possibly other uninfected home worlds of Tier 7 races they considered "worthy".

The Forerunners employed many, many measures, including intricate naval tactics in an attempt to contain the Flood, which ultimately failed. Finally, the Forerunners were forced to a Pyrrhic solution, building seven ring-like super-weapons, known as Fortress Worlds or Halos, across the galaxy. Because the Flood were parasitic, and their survival was directly linked to the presence of potential hosts, the Forerunners reasoned that eliminating all potential Flood hosts, that is, all sentient life forms in the galaxy, would "render the parasite harmless", as explained by the Forerunner AI 343 Guilty Spark. After all countermeasures failed, the Forerunners activated the Halos in 97,448 B.C, unleashing galactic destruction, and destroying almost all sentient life forms in the Galaxy. However, the human race and many other species throughout the galaxy survived the firing of the Halos because they were safely spared on the Ark which was out of range of all seven Halo Rings or protected by the portal, which was also out of range of all seven Halo Rings.

The Flood were contained, and unable to reproduce due to lack of hosts, and eventually died out. The only surviving specimens were contained in state of the art, high-security Forerunner research facilities, such as the Halo Installations and the Threshold Gas Mine.

Battle of the Apex
Thousands of years after the end of the Forerunner-Flood war, the human ship Spirit of Fire jumped from Arcadia to follow the signal of Professor Anders, who had been captured by the Covenant. They were led to an unknown planet, which happened to be a Forerunner Shield World. Ground forces were deployed to find out about unknown hostiles on the ship's radar. After killing many elites, Sergeant Forge and several Marine squads were attacked by the Flood.

The ship's crew was tasked to rescue fellow Spartans that were surrounded by Flood forms. After one was rescued, the small squad made their way into a site that would become a base. They then got a larger force to attempt to rescue the last. Once it was rescued, the squad returned to the base to fight the new hostile, the Flood.

SPARTAN-II Red Team, while searching for Anders, was ambushed by several Flood forms, which forced their Warthog to crash. The remaining forces at the nearby base, led by Sergeant Forge, moved out to rescue the ill-fated team. As they attempted to rescue the SPARTANS, they soon discovered that the Flood was creating a Proto-Gravemind, and that it may have been jamming the tracking signal that Anders was giving out. On their way to the soon-to-be Gravemind, they encountered several Flood Colony forms, and found that by destroying them, the Proto-Gravemind would become weakened. Once they destroyed the Proto-Gravemind, Serina found a signal, which was believed to be Anders'.

Captain Cutter then ordered all UNSC personnel on the planet to return to the Spirit of Fire. These orders were impeded by the Flood, which attempted to prevent the humans' retreat, but the Flood were eventually held back long enough for the UNSC forces to return to the Spirit. The Flood attacked the Spirit as it entered the interior of the Shield World, but were wiped off by the ship's crew, Sentinels, and the Shield World's cleansing rings. All Flood on the Shield World were destroyed when Sergeant Forge used the Spirit of Fire's Shaw-Fujikawa Translight Engine to destroy the Shield World's internal sun, obliterating all Covenant and Flood forces on the Shield World. The Spirit of Fire narrowly escaped by performing a hastily-calculated slingshot maneuver around the exploding sun in order to shoot off into space. Without the Spirit's Slipspace Engine, however, the ship was lost in space, with humanity's only knowledge of the Flood lost with it.

Release
Although the first firing of the Halo Array starved large numbers of the Flood, samples of parasitic specimens were kept alive (perhaps in stasis) on various Forerunner outposts for research and further scientific study. One such research site was located on Installation 04. This proved to be a grave, irreversible mistake, as both Humanity and the Covenant stumbled upon the abandoned ring thousands of years after the firing of the Array and the destruction of the Forerunners and most of the Flood. Cortana, aboard the UNSC Pillar of Autumn, made what was presumed to be a "blind" jump through Slipspace (although it was later revealed that she used translated co-ordinates from a Forerunner artifact found on Sigma Octanus IV) to evade Covenant pursuers. This led both species to Installation 04, and in turn, the Flood that were kept there. After the crash of the Pillar of Autumn on the Halo Ring, the surviving crew members dug in for what they believed would be a long and brutal guerrilla campaign against the Covenant forces on Halo.

Meanwhile, the pursuing Covenant cruisers (among them the Truth and Reconciliation) amassed their ground forces on the surface, and soon discovered an underground Forerunner Flood Containment Facility in the swamps of Halo. Due to the secure nature of the facility, the Covenant came to use it as a fortified base of operations and an area for storing weaponry, oblivious to its true nature. Though some of the Flood managed to escape, the terrified Covenant soldiers eventually pushed the Flood back in. It is unknown whether a few still escaped. The Flood escaped due to a leak in the library, allowing the flood to reproduce by spores.

After Captain Jacob Keyes was captured by the Covenant for interrogation, he caught wind of this weapons cache and, upon his rescue, organized Fire Team Charlie to seize it. When the team arrived at the swamp however, they found the facility was completely devoid of Covenant personnel. However, upon takeoff, a Pelican Dropship, whose call sign was Victor 933, which had transported them to the base, crashed into the swamp as a result of enemy action. Then, immediately after the crash, the pilots were able to record a message about their attackers, calling them "Unknown Hostiles". These "Unknown Hostiles" were actually the Flood, and shortly afterward, they could be seen creeping throughout the swamp, as witnessed by John-117 when he was deployed into the area. At first, when he saw the Covenant retreating out of the Forerunner structure running from Assault Rifle fire, he believed them to be allied forces. However, after he opened the door that Captain Keyes had unlocked, he learned the truth through Private Jenkins' recording: this was no Covenant weapons cache. He then proceeded to escape, escorted the surviving marines to a drop zone, and encountered 343 Guilty Spark. Guilty Spark then promptly teleported him into the Library and said, "The Flood is spreading. We must hurry!"

The few surviving Covenant soldiers left were scattered and disorganized, and put up little resistance to the new Combat Forms created from numerous Covenant corpses. It is presumed that the Covenant occupants of the research facility had moved through the structure in hopes of finding Forerunner artifacts, and had inadvertently released living Flood specimens. Knowing full well what the Flood are capable of, the Covenant forces promptly locked the facility down and evacuated all personnel who survived the Flood's escape. Keyes and his team were unaware of this fact, however, and proceeded to delve deeper into the facility, ultimately coming across one of the unopened Flood Containment Chambers. Believing it was a weapons cache of some sort, they unlocked the door, and were promptly attacked and assimilated by the Flood inside. With a large supply of "edible" sentient life now occupying the ring, the Flood began to multiply and infest most of Halo. Soon, the infestation proved too great even for Halo's automated military force, the Sentinels, to handle, and the ring's commanding AI, 343 Guilty Spark, was forced to call upon a "Reclaimer" (John-117) to activate the array once more. Hearing of the above events, John-117 was deployed to the area on the drop ship Echo 419, proceeding into the facility, only to find out that most of the Marines he had been assigned to rescue had succumbed to the Flood.

These events brought the knowledge of the Flood to the UNSC. In a war of losses, this was another grim revelation for humanity.

Assault
After the Flood's escape, the Covenant forces, who were widely stretched across Installation 04, were heavily engaged, and were being overwhelmed by Flood forces. While at places, the Covenant were able to hold the line by using vehicular and air support, most of the weaker Covenant soldiers such as Grunts and Jackals succumbed easily to the Flood, and the few Elites who remained were outnumbered and destroyed. The UNSC, having a much smaller presence on the Ring, found out about the Flood's might a short while later, when a UNSC Orbital Drop Shock Trooper force engaged with the Covenant near Alpha Base, was ambushed by the Flood and suffered heavy casualties. However, one of the attacking combat forms, Wallace A. Jenkins, who was one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by a weak and elderly infection form whose powers of infection had been dulled by years of hibernation, making Jenkins' transformation into a combat form incomplete. Jenkins was still able to resist the pervasive Flood entity in his mind, and although incapable of articulate speech and with a physiologically devastated body, Jenkins was still able to betray the Flood through hand gestures during an interrogation with Major Silva, informing the UNSC Marine battalion that the Flood planned to assault the UNSC base from a series of underground catacombs underneath their base. In response to this intelligence, the UNSC moved to secure them, and prevented the Flood from staging another attack.

By the time of these developments, Flood dominance over the Covenant was assured. The Sentinels were unable to contain the Flood, and the Covenant and UNSC were sustaining heavy losses and rapidly losing ground. Meanwhile, the Flood were occupying whole areas formerly held by the Covenant, although Covenant and Flood forces continued to wage war on the massive ice plains of the Ring. Although the Covenant had the benefit of Shade stationary plasma turrets and vehicular/aerial support, the Flood had captured both UNSC and Covenant weapons, most notably M19 Rocket Launchers and massive numbers on their side.

The Flood, however, were not unintelligent. One of the UNSC personnel captured in the Marine force to the ex-Covenant arms cache was Captain Jacob Keyes, a brilliant naval strategist of the UNSC Navy. When he was infected, instead of using him in combat, the parasitic infection form, while deleting his memories, felt that Keyes held vital information. Keyes knew the location of Earth, the birthplace and major planet of the human race, a planet with tremendous possible assimilation opportunities for the Flood. Instead of becoming a combat form, he was merged with at least four other victims into a massive, engorged Proto-Gravemind aboard the CCS-class Battlecruiser, Truth and Reconciliation, a Covenant warship that the Flood were relentlessly attacking. While he was in the brain form state, the Flood began to comb his memories. Captain Keyes successfully withheld information about Earth by constantly accessing information available on his Command Neural Interface such as his name, rank, and serial number, until the Chief arrived. However, his arrival was too late, for Keyes was already dead. It was at that time the SPARTAN-117 punched a hole in Keyes' skull to get the Captain's CNI.

The Covenant Flagship Truth and Reconciliation was a major point of contention between the Covenant Special Operations units and the Flood. The Flood had likely entered the warship previously, decimating most of the Covenant crew and garrison on board, leaving only isolated pockets of resistance, mostly small numbers of terrified Grunts and a few Elites aboard the vessel. However, below the Truth and Reconciliation, in a series of rocky canyons, the Covenant were much stronger in force, with large numbers of Elites augmented by Grunts, Jackals, and Hunters, fighting off the Flood. The Covenant High Command on Installation 04, however, were terrified by the Flood presence. Because the CCS-class battle cruiser had been damaged in a previous space battle with the UNSC Pillar of Autumn, it had been grounded, awaiting repairs for at least two days. After The Flood were discovered to be aboard the battle cruiser, the Covenant High Command sent a Special Operations strike force to the Truth and Reconciliation to commence immediate emergency repairs, prepare it for lift-off into space and neutralize The Flood. These efforts, however, failed, with the Flood eventually retaking the vessel. Later, the remaining UNSC Marine forces on the ring staged an assault on the Flood-held Truth and Reconciliation, and managed to neutralize most Flood forms on the cruiser in a swift assault, and prepared the battle cruiser for take-off with emergency repairs to escape from the ring to Earth. However, there was tremendous risk if the Flood-infected vessel traveled to Earth, if there was even one Flood carrier form aboard, "Earth could fall," according to UNSC AI Wellsley, because of the Flood's exponential parasitic potential. Against orders, Marine officer McKay destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel, including an infected Jenkins, but saving Earth and possibly billions of lives from the Flood. The fact that a single Flood form could cause such destruction has been later supported by Rtas' Vadum when he lectures Lord Hood: "One single Flood spore can destroy an entire species."

Defeat
The Flood appeared to be interested in seizing space-capable vessels, in the hope of spreading itself away from the Installations and into the stars: they attacked the UNSC Pillar of Autumn and CCS Truth and Reconciliation, for this reason. After the UNSC cruiser's crash-landing onto the ring, and after engaging over fifteen enemy warships, it had been taken by the Covenant, although a UNSC raid had managed to temporarily retake the hull. On the third and final day of the Battle of Installation 04, however, the Covenant were in control of the crashed warship when a Covenant patrol was ambushed by the Flood and infected; when it returned into the vessel, the Covenant garrison was attacked by Elite Combat forms. The combat forms were able to scatter before they could all be destroyed, and a steady trickle of Flood continued to enter the Pillar of Autumn through vents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force on board. Eventually, the Sentinels came on board, overwhelming the dwindling Covenant forces as the Flood began to attack the warship in full force, in the end almost completely obliterating the Covenant presence, and severely limiting the Sentinels on board. Despite their best efforts and enormous numbers, the Flood, Covenant, and Sentinels were unable to stop John-117 from detonating the wrecked warship's reactors, and escaping in a C709 Longsword-class Interceptor. The resulting thermonuclear detonation severed the ring world, destroying it and its capability to annihilate all sentient life, and simultaneously killed all remaining Flood on Installation 04, along with any other life forms that were on the ring.

Threshold Gas Mine
The Flood were also kept in the Forerunner-constructed Threshold Gas Mine, in one of the three peripheral arms of the station. A small Covenant special forces strike team of three Phantoms laden with Special Operations Elites and Grunts, led by Special Operations Commander Rtas 'Vadumee and Thel 'Vadamee, were dispatched by the High Prophets. This was to "silence" Sesa 'Refumee, the leader of the Heretics stationed within the mine encouraging dissent against the Prophets. While the force attempted to track down Sesa 'Refumee to assassinate him and destroy his forces, the Heretics, unleashed the Flood that were held within the station, and there was absolutely nothing they could do about it. During Thel 'Vadamee's pursuit of Sesa 'Refumee, where Thel 'Vadamee neutralized several weapons emplacements. Enemy Banshees, Thel 'Vadamee, Rtas 'Vadumee, two Spec Ops Elites, and two Spec Ops Grunts entered into a laboratory, and the Heretics deliberately locked the doors: most likely by design, in order to crush the Special Operations force. The Heretics then released the Flood from containment in that arm of the station, locking Thel 'Vadamee and his five allies into a peripheral laboratory, and then besieging them with Infection Forms while Heretics dueled the Flood underneath them. When that trap failed to annihilate them, instead dealing tremendous damage to the would-be trappers, 'Vadumee retreated to call for reinforcements and coordinate a continued assault while the Arbiter and any remaining allies pursued 'Refumee to a gigantic elevator that ran in the spine of the Flood-infested arm: while presumably used to transport gas conduits, its walls were rampant with ravenous Flood, and it ferried canisters full of Infection Forms. The tenuous, agonizing elevator was nicknamed "A Descent Into Hell". Thel Vadamee's allies were slowly picked off as Flood attacked the elevator, with Sentinels continually hovering overhead and firing lasers into the chaotic fray. Finally, Thel 'Vadamee left the elevator into a second wide laboratory hall, where the Heretics attempted to lock out both Thel 'Vadamee and the Flood. The Heretics' plan was not flawless, however, and handful of Heretic Sangheili were still present in the second laboratory when the Flood entered, these Sangheili were presumably infected while a horrid tide of Infection Forms, Combat Forms, and Carrier Forms lurched into the hall, and Thel 'Vadamee was forced to defend himself against them until the tide stilled and the doors were opened.

The Flood could not be contained in the rooms, however, and attacked both the Covenant assassination force and the Heretics themselves. Their assault was too much for either the Heretics or their attendant Sentinels to contain, and the Flood passed on a pathway between their infested peripheral arm to the central core of the station, where they overwhelmed the Heretics there. Thel 'Vadamee continued to pursue Sesa 'Refumee, but was impeded by large waves of Flood, crushing the outnumbered Heretic Sangheili, Heretic Unggoys, and the ever-dwindling number of Sentinels. Eventually, the Sentinel presence was completely obliterated as the Flood spread to the upper levels of the station. However, Thel 'Vadamee managed to sever the cables that secured the gas mine to its support structure higher in the atmosphere of the gas giant. This caused it to plummet into the maelstrom as the Arbiter tracked down and eliminated the Heretic Leader. While Thel 'Vadamee and 'Refumee were engaged in battle, the Monitor 343 Guilty Spark explained that the act of cutting the cables was a fail safe in case of a Flood outbreak, and that any remaining Flood (and, presumably, remaining Heretics) would be killed once the facility entered the planet's solid Nitrogen core.

Battle of Installation 05
Unlike on Installation 04, where the Sentinels had contained the Flood until the Covenant began to meddle with the Forerunner containment measures, on Installation 05, the Flood had been in continual war with the Sentinels since the firing of the Halos 100,000 years ago. Even up to 2552, the Flood were presumably dwindling with no fresh supply of hosts (perhaps they were involved more in hiding than in open warfare). However, like on Installation 04, the fresh suitable host bodies of a newly-arrived Covenant armada and a UNSC strike force fueled the Flood's exponential parasitic capabilities, and Flood infestation increased at a surprisingly rapid rate, and within hours, Sentinel forces were being overrun, causing a risk that the Ring would fall to the parasite.

In order for the activation of the Halos and the facilitation of the ultimate Flood containment measure to occur, a Reclaimer had to retrieve the Index from Installation 05's own Library, making the Library a high-priority target for the Flood: if it could control the Library and prevent any Reclaimer from entering, it would make itself essentially invulnerable. Installation 05's Library was nestled in a tenuous series of wintry and snowy canyons and boasted three levels of defense: an exterior Sentinel Wall kilometers in the distance, an underground honeycombed facility that produced large numbers of Sentinels (that impeded any invaders from advancing), a Containment Shield (an energy shield that denied access by any means to the Library until it was deactivated), and a Quarantine Zone (a type of biological "airlock" to prevent the Flood from spreading any further). This multi-tiered defense was patrolled by hundreds of Sentinels and Sentinel Majors, and several Enforcers.

However, two external forces, the Covenant and the UNSC, wanted the Index as well: the Covenant, to activate the Halos and set off the misinterpreted "sublimation event" of the Great Journey, and the UNSC to secure the Index and prevent the Covenant from doing so. The Covenant sent a Strike Team of Special Operations Elites, commanded by Rtas 'Vadumee, and led into battle by the Thel 'Vadamee, to retrieve the Index in a multi-staged assault. However, the Covenant first needed to deactivate the Containment Shield by disconnecting its power generator in the center of the Sentinel Wall. Previously, a force of Jiralhanaes, Unggoys, and Kig-Yars had been dispatched to complete that operation, but they were overwhelmed by Sentinel defenders and forced into a defensive position instead of an attacking one. Thel 'Vadamee was personally ordered by the Prophet of Truth to find and retrieve the "Sacred Icon" (the Covenant term for the "Index") for the Covenant. Because the Sentinels activated the Containment Shield, he could not be directly air-dropped to the Library; instead, he had to land at the periphery of the Sentinel Wall, disable the generator, and then proceed to deactivate the generator. Tartarus deployed Thel 'Vadamee in a Phantom drop ship, where Thel 'Vadamee entered the first half of the Sentinel Wall, where there was evidence of heavy Covenant casualties and abandoned infrastructure, making his way down through the facility and finally deactivating the Containment Shield. While the first half of the Sentinel Wall had remained pristine, the second half, the one buttressing the Quarantine Zone, was infested by Flood. Sentinel resistance had been destroyed in some segments of the Flood-Infested Wall, and in heavily infested areas, the Flood appeared to be beginning to terraform the Forerunner facility, spreading murky spores into the air. The UNSC deployed a small number of Marines to fight through the Flood-Infested Wall, but they were quickly overrun. Thel 'Vadamee was able to battle through hundreds of Flood forms that had taken up positions in the Wall, which had entered through wall-mounted conduits, although later, the Covenant deployed a team of three Special Operations Sangheilis in the lower levels of the Flood-infested Wall, although they were killed as well. Finally, Thel 'Vadamee was able to make his way out of the Wall, emerging at the ground levels: the Quarantine Zone.

With the Sentinel Wall compromised by the Covenant and the Flood, and the Containment Shield lowered by Thel 'Vadamee, there was only one last line of Forerunner defenses: the Quarantine Zone. There was a significantly elevated number of Enforcers present, even automated sentinel construction factories flying over watch, and automated factories built into the landscape, each churning out Enforcers and Sentinels, further swelling their numbers even in intense combat. Here, the Flood demonstrated their unusual technical aptitude, commandeering Covenant and UNSC vehicles, possessing dozens of Ghosts, Wraiths, Warthogs, and Scorpions to do battle with the hovering Enforcers. Flood forces also boasted large numbers of Combat Forms, which appeared to infiltrate the Containment Zone by means of gaping chasms that periodically split the ground, emitting a green mist and appearing to be of indefinite depth, perhaps linkages to the Flood-infested underground.

Six Special Operations Elites, along with Rtas 'Vadumee, were airdropped at the periphery of the Quarantine Zone by means of Covenant drop pods, and Thel 'Vadamee joined them, rallying their efforts to secure a landing zone against large numbers of Flood attackers. Using the advantages of elevated ground, Shielded Plasma Cannons, and a Deployable Lookout Tower, the small Special Operations contingent was able to hold an LZ for a Phantom to drop off a Spectre and a pair of Ghosts. Meanwhile, 'Vadumee commanded his men to assist Thel in forging towards the center of the Flood-infested Quarantine Zone, so that Thel 'Vadamee could reach the Library and secure the Sacred Icon.

The Flood, despite a significant vehicular and infantry presence in the Quarantine Zone surrounding the Library, were unable to deny access to the Covenant and the UNSC. Miranda Keyes managed to slip the In Amber Clad through the Forerunner Enforcer patrols, while the Covenant launched a ground borne attack, led by Thel 'Vadamee, through the battling Flood and Sentinels. Later, In Amber Clad was captured by the Flood.

Attack On High Charity
Spartan-117, who had arrived with Keyes and Johnson, met Thel 'Vadamee for the first time in the clutches of the Gravemind, the central controlling intelligence of the Flood hive, created from the combined intellect of tens of thousands of assimilated corpses. Gravemind attempted to convince Thel 'Vadamee, against his own beliefs, and through the use of the captured Monitor, 2401 Penitent Tangent, and the semi-revived Prophet of Regret to not activate the ring. Still unswayed, although in question of his religion, Thel 'Vadamee prompted the Gravemind to show him, by "Stopping the key from turning.", transporting Spartan-117 to High Charity, and Thel 'Vadamee to Delta Halo's Control Room.

Taking advantage of the distraction that Spartan-117's presence caused, as well as the ensuing Covenant Civil War, Gravemind took the opportunity to deliver himself, as well as a multitude of Flood underlings, onto In Amber Clad and then jumped the UNSC ship inside High Charity, whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled Pelican Dropships delivered Flood all over the city. The ill-equipped and distracted populace was no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the Prophet of Mercy falling victim to an infection form.

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems; the whole city was becoming Terra-formed into a massive Flood Hive. The Gravemind regularly rebutted the words of comfort the Prophet of Truth delivered to the city’s population over the comm system, to sadistically instill more panic and unease. Spartan-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the Dreadnought that was leading the Prophet of Truth's fleet to Earth. Cortana, who had to stay behind to manage the situation should Installation 05 be fired, was left in the clutches of the Gravemind, who could potentially learn all possible Human knowledge from her.



Earth, The Ark, And The Flood's Defeat


""Child of my enemy, why have you come? I offer no forgiveness... a father's sins pass to his son.""

- Gravemind on the level Cortana

""I am a timeless chorus. Join your voice with mine, and sing victory everlasting.""

- Gravemind on the level Floodgate

""Of course, you came for her... We exist together now, two corpses, in one grave.""

- Gravemind on the level Cortana

At the conclusion of the Battle of Voi, with the Prophet of Truth activating the Portal and the Jiralhanae fleet escaping to the Ark, a Flood infested Covenant cruiser appeared out of Slipspace, having penetrated the Elite's blockade of High Charity and crash landed into the city of Voi in an effort to spread. In order to prevent the Flood from escaping and infecting the entire planet, Miranda Keyes ordered John-117 to detonate the cruiser's reactors and search the ship for Cortana, causing the city of Voi to be destroyed and the Flood there eliminated. As John-117 and the Thel 'Vadam fought their way to the ship, the Shadow of Intent arrived and deployed Sangheili task force to aid UNSC marines. SPARTAN-117, with the help of the Sangheili, was able to battle through the Flood forces, including the newly encountered Flood Pure Forms to reach the crashed cruiser where he found a message from Cortana. Upon this discovery, 343 Guilty Spark arrived pledging his assistance to the Reclaimers. With the Flood spreading, SPARTAN-117 boarded a Phantom while the Sangheili went on to glass the infected areas. The remaining Sangheili forces, fulfilling their mission of removing the Flood from Earth, retreated back to the Sangheili Carrier, the Shadow of Intent, along with Cortana's message and Guilty Spark.

With the assistance of 343 Guilty Spark, Cortana's message was repaired, revealing a solution to the Flood menace on the Ark. To Lord Hood's disapproval, Miranda Keyes led UNSC forces allied with Thel 'Vadam's and Rtas 'Vadum's Sangheili's through the Portal.

On the Ark, while attempting to lower the shield surrounding the Ark's citadel, the infested High Charity made a Slipspace jump and smashed into the Ark, releasing the Flood inside the structure and causing an infestation on the newly built replacement for the destroyed Installation 04. After retrieving Cortana from the derelict High Charity, SPARTAN-117 was able to kill all Flood present on the Ark and the replacement Installation 04 by activating the unfinished Halo using the Index Cortana retained from the first Halo. The ring's activation also presumably killed the Gravemind. SPARTAN-117 also destroyed High Charity by destroying its reactors, causing them to go critical and killing the billions of Flood that were present when High Charity was destroyed, in a huge explosion.

Aftermath
With the Gravemind destroyed, and all the Flood on the Ark gone, it is probable that the Flood still exist on the other Halo rings, excluding Installation 05 (although it is possible that all of the Flood on Installation 05 didn't leave and there are still some there, since Installation 05 was never destroyed), or any other remaining Forerunner Installations. The fact that the Flood were never originally from this galaxy further complicates the issue, and how they arrived leads to more possibilities. It may be that is why the Forerunners preserved some Flood for research on the Halo rings instead of destroying them all, since the Flood would return sooner or later, and it would be better to fight a researched enemy rather than a foreign one. At least for the moment, the Flood are seemingly defeated.

Developmental Stages
The Flood go through four distinct developmental changes.
 * 1) Feral stage is the lowest form: communicating via pheromones, and have the natural instinct to harvest enough calcium to establish a viable Gravemind. (Seen in Halo: Combat Evolved and Halo Wars)
 * 2) Coordinated stage is when the Flood become truly dangerous; they now take orders from the Gravemind that was created in the first stage. (Seen in Halo 2)
 * 3) Interstellar stage allows the Flood to take control of most technology and spread throughout the galaxy to infect more hosts. (Seen in Halo 3)
 * 4) Intergalactic is the last known stage the Flood enters, theoretically, in which it utilizes all captured technology to depart to uninfected galaxies to further replicate. It is assumed that the Flood that had arrived in the Milky Way had descended from Intergalactic level Flood. (Seen in the Forerunner-Flood War).

Biology


The Flood, a virulent species of parasites, is an anomaly in all known biology, and an anomaly to regular life. Due to this, it is thought to be an extragalactic species, originating from different galaxies, and even from different universes. It is extremely adaptive, and mutates hosts through different stages of accelerated "evolution", where the body of biomatter accelerates its own evolution to the point of a massive biological breakdown, where by the infection can first take place. The virus can reproduce through asexual methods, though this is described as a last ditch effort, and only performed by pure flood forms.

The only known pathway for Flood reproduction and survival is by the infestation and assimilation of other species. They do this through a form of cellular reproduction known as lysogenic replication where the viral cells infect host cells with their own genetic code which mutates the host cells into viral forms.

Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater (in early stages of life), the Flood can withstand harsh environments of all known colonized planets.

It would seem that the Flood can infest any living organism, but that they prefer hosts with a large brain capacity to accelerate the formation and expansion of the Gravemind hive intelligence and the Flood intelligence as a whole. They are only as intelligent as the beings they infect; the more cunning the enemy, the more dangerous the Flood will be.

As the Flood spread and replicate, they will alter the environment to better suit their needs. This takes the form of biomass growing on any available surface, and the air being filled with spores. This appears to be part of the Flood life cycle and is done automatically, and leads to the development of a Gravemind and a Flood Hive.

The flood seem to thrive in moist and humid areas, which lends itself to the fact the the Quarantine Zone is bereft of organic life, and is cold and perpetually snowy, to hinder the development of the flood.

Neurological
Infection forms selectively target other species that possess sentient intelligence and are of sufficient biomass, and can infest living or lightly wounded dead bodies. Infection forms have long tendrils that pierce the skin of the host and find their way to the spinal cord. From here they synchronize with the host, matching frequencies with the host's nervous system and taking over brain functions, completely rewriting the psychology of the creature to the ravenous and voracious psyche of The Flood, although retaining higher-level cognitive abilities, such as combat techniques and technical knowledge which may be useful. However, resistance of the host is completely destroyed, as the original mind's memories are systematically erased, leaving only a simplistic and primal urge to assimilate other species and destroy them. During the early stages of infestation, when the host still retains its original consciousness, there are distinct somatosensory sensations: first the host's extremities grow numb and their lungs feel heavy. Then, the greater Flood intelligence begins to take over the mind, and sensation becomes increasingly indistinct: the host's vision seems to be "A gauzy white haze", but the host can make out shapes and the host can hear a low droning through the ordeal. Finally, once physiological mutilation occurs, the host can vaguely feel dramatic changes to its own body: the limbs feel to the host as if they are made of a spongy liquid and the lungs itch as well. However, the host can still control their limbs to some extent if the neurological contact with the infection form is weakened. They can use the vocal cords of Jiralhanae's, Humans, and Sangheili's to communicate as well as the moans they use as observed on The Covenant where the drop pod crashes into the third shield tower. If you hide behind one of the spires you can sometimes hear them say things such as: "Where is he?"(Uncommon), "Don't hide from me!"(Common), "You will be ours!"(Occasionally), "No skill." (IWHBYD skull required), "Get out of there!" (Occasionally) and "No obstacles will stop us...No retreating will halt us!" (Rare). It is likely that this is Gravemind speaking through the infected hosts, as he does this just before you team up with him towards the end of The Covenant. Gravemind can also speak through Tank Forms. However, it is unknown how he does this since they are Pure Flood matter, unless the Flood engineered Tank Forms to have vocal chords.

Physiological
While baseline neurological assimilation is the same regardless of host species, The Flood's infestation includes physiological mutilation, which is species-specific.

Higher-level species capable of combat, such as Humans, Jiralhanae, or Sangheili are selectively infected to become Combat Forms. Combat Forms retain the general profile of their original species, although they are significantly mutated. Organ-based systems' physiology are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible; organ functions are decentralized, thus, decapitation does not down a combat form. Furthermore, highly area-specific munitions, such as sniper rifle projectiles, are completely ineffective, as picking off selective areas of a combat form leads to no wide-scale physiological impediments, and the projectile will punch through the decayed flesh rapidly, exiting through the other side of the combat form without consequence. Combat forms also generate tentacles very quickly without regard for the skeletal structure of the host, with tentacles protruding from the combat form at odd locations, typically localized to the chest axial regions, such as the neck, armpits, or torso. A single arm is oftentimes mutated to a claw like or tentacle-like structure that splits into multiple angular, hand-like structures replacing the hand's usage, yet it will still be there. The musculature of the combat form dramatically changes as well, and the form is given prodigious arm and leg strength, allowing for heavy blows with its arm-like structures and for incredible speed and jumping height with its legs. The Flood are also prodigious climbers, able to scamper on walls with little hindrance to the organism. Metabolic requirements, however, appear to be extreme on the organism: blood vessels become extremely prominent on the surface of the Combat Form, with thick and distinct blood-colored vessels spawning all across the creature's surface, most prominently so on the head.

Countermeasures
There is no confirmed way to effectively stop a Flood infection when an Infection Form attacks a host. However, Two possible exceptions are known. One of which is to contract Boren's Syndrome, a neurological condition where radiological exposure causes electrical anomalies in the host nervous system. The neural frequencies of the host are scrambled, although without major negative manifestations to the host. However, it has been suggested that during Flood infection, the infection form is unable to tap into the host's nervous system to kill them. It is unable to proceed to a large-scale physiological mutation, not understanding why. Although Flood DNA is sliced into host DNA, even imparting the host with increased regenerative capabilities. Avery J. Johnson, having Boren's Syndrome, escaped the containment facility where The Flood attacked two squads of UNSC Marines. There is some reason to suspect that this was due to his past in the ORION Project, and that the Boren's Syndrome explanation was invented to hide it. The truth in this matter remains uncertain. In the novel Halo: The Flood, Cortana uses an electrical charge which she generates from Master Chief's suit to destroy an infection form that was attempting to infect him.

While Boren's Syndrome left untreated may leave the victim immune to the Flood, leaving it untreated usually equates to death within a couple of years. There is said to be a one in a billion chance that research, from someone so inflicted with Boren's Syndrome in a way that they cannot be infected, could be carried on to produce a cure.

Since the Flood is a parasitic life form, they depend on other organisms for survival. Thus, the only logical foolproof way to eliminate them permanently is to take away their food sources. The Forerunners attempted this by bombing large population centers, but this was seen as one of their last resorts and as such was performed too late in the war to make much difference. The absolute last resort was the use of the Halo installations, which effectively destroyed all life in the galaxy and starved the Flood. However, the Forerunners' plan included some measures to preserve sentient life and return them to their home planets, allowing the Flood's "food source" to persist, while at the same time specimens were kept for analysis on Halo Installations, allowing the threat of the Flood to be merely forestalled, not eradicated.

Environmental stimuli seem to have an effect on Flood metabolism. Halo installations typically artificially incorporate low temperature into environments suffering Flood outbreak and to create natural obstacles to Flood progress, regarded as Quarantine zones. At least one Flood Research Facility used low temperatures to keep its specimens inactive and dormant as it studied them. This may be the reasoning behind keeping Installation Control Rooms in low degree weather, impeding their occupation by the Flood. The Flood are also vulnerable to temperatures on the other end of the spectrum. Plasma weapons will burn through them, as well as Sentinel Beams, indicating that the Forerunners had determined energy weapons to be the best 'basic countermeasure' available to fight the Flood. The Flamethrower is also devastating to all forms of Flood, further indicating this weakness. It may be a side-effect of the Flood's conversion of the body into biomass, breaking down cellular integrity and increasing flammability.

Flood Forms
The basic Flood form is the infection form, unique in its distinctive spherical shape. The infection form is used to infect organisms of other species, physically adhering to their surfaces and inserting tentacles, forcibly "hacking" into its host's nerve system, completely changing its psychology to that of the Flood's, and eventually physiologically mutating it into one of the various parasitic Flood forms of whomever the victim may be. There are 7 direct, Flood only, forms.

Flood Spores
Flood Spores are the most simple Flood organism, facilitating the creation of infection forms out of available biological material to advance infestation. These are small airborne contaminants, capable of terraforming the surrounding environment.

Flood Infection Form


The Infection Form is the second most basic and simple form of the Flood, and is larger only than the Flood Spores, and thus the easiest to defeat. It is believed to be the most essential form to the flood, as it is the Flood's most common method of infecting and assimilating other species, as these infections attack in dozens or even hundreds at a time. Interestingly, the infection forms pop like balloons when you destroy them in Halo: CE.

Flood Combat Form


After an Infection Form has successfully assimilated a sentient creature with sufficient biomass and combat potential, it will neurologically and physiologically mutate the host, twisting it into a Combat Form. Combat Forms oftentimes maintain the general profile of the original host organism, and possess the original clothing and armor they were wearing before infection. Infection forms have been seen mutating Humans, Elites, Brutes, Grunts, and Jackals into combat forms. Hunters cannot be infected due to their lack of sufficient calcium and a central nervous system. Also, Drones cannot be infected either, since they also lack protein, and a basic skeleton in which the Flood craft their biomass into.

When a Combat form has served its usefulness, the infection form may dislodge itself from the body and look for a new host, leaving the body for use as spare biomass. Alternatively, the Combat form could be made into a Carrier form.

Flood Carrier Form


The exact origin of Carrier Forms is unknown, although they are theorized to be former combat forms that are too damaged or too aged to be useful for future combat. The purpose of a Carrier Form is to nurture and disseminate Infection forms upon attack or proximity to a host.

Pure Forms
This type of Flood is first featured in Halo 3. As implied by their name, Pure Forms are not the product of infecting a biological host, but are instead composed entirely of Flood biomass. They are a new stronger strain of Flood that develops once a coordinated stage has secured sufficient resources, namely in the form of calcium from host species. The Flood make use of those reservoirs of calcium to develop an adaptable framework upon which the Flood Super Cell is attached, which creates a fearsome biological war machine. Initially, this form's primary function is to aid Infection Forms by securing more biomass in the form of hosts, as well as enough complex neurological systems to strengthen the local Gravemind. This duty overlaps with the Combat Form, but once enough Pure Forms have developed, the Combat Forms are relegated to defensive roles, or adding to the biomass and calcium reservoirs. Flood Stalker forms, Tank forms, and Ranged forms are Flood Pure forms that appear in Halo 3. Flood Transport forms, Infector forms, and Stealth forms are Pure forms that were cut from the final version of Halo 3. Flood Bomber forms, Swarm forms, and Thrasher forms could possibly be Pure forms, but this is unlikely. The Pure Forms also bear a lot of resemblance to the Mgalekgolo and use the same animations for melee's.

Flood Stalker Forms
Stalker forms are extremely agile and can jump over large distances. They tend to crawl on ceilings and walls before leaping at an opposing enemy to attack. A stalker form usually turns into a tank form when a target gets close to it, and a ranged form if you're shooting at it and a wall is nearby. Ranged forms mainly turn back into stalkers before transforming into tank forms.

Flood Ranged Forms
Ranged Forms can usually be found clinging to ceilings or walls, though unlike the Stalker Form, they rarely move from a given spot. They can fire sharp projectiles at you, without the need of a weapon. The ranged form is susceptible to the Covenant Needler. Plasma Rifles are also effective against them, due to their high ammo capacity, relative accuracy, and lack of reloading. It is especially weak against plasma, spike, and firebomb grenades, though they don't usually have a long range. The ranged form can also fold itself while remaining on a wall; when folded, it is much harder to kill. Also the spikes may look familiar because they are similar to spikes from a Brute Spiker. Sometimes, when on Legendary difficulty, the Ranged Form will drop Spike Grenades, and like Spike Grenades, their projectiles do no damage to Deployable Cover, making it an ideal defense against them.

Flood Tank Forms
Tank Forms are large and easily spotted. Unlike the other Flood Pure Forms, they cannot climb on walls and ceilings. These Flood can take a tremendous amount of damage. They also occasionally spew Infection Forms from their 'mouths'. Tank forms may be mistaken for a (nonexistent) Hunter form, mainly because a tank form has one over sized hand (equivalent to a Hunter's shield), a small arm (equivalent to where a Hunter's gun would be), its roar which sounds similar to that of a Hunter, and even more similar body mass. It is however, impossible for a Hunter to be infected (despite these similarities) due to the fact that Hunters are a colony of invertebrate worms lacking bones (i.e. calcium) and with insufficient biomass per worm for an infection form to modify. There is speculation that it is a revised form of the Flood Juggernaut.

Flood Prophet Form
Prophet Forms are rare products of infestation. Because of this, many mechanisms and details of Prophet Forms are unknown, although it appears that the Prophet Form is directly infected and incorporated into one of the many tentacles of a Gravemind form, and fully retain the original voice and full intelligence of the former Prophet.

The Prophet Form is like the Combat Form in the physiological distinctions it shares with the original organism before infection: Prophet Forms can easily be compared to Prophets by their heads and, to a lesser extent, their chests, merely having a central body of the Gravemind tentacle. Only one infestation process of the Prophet form is seen, but yet to be completed (High Prophet of Truth in Halo 3).

Brain Form/"Proto-Gravemind"
A very rare form of flood is the Brain Form (a.k.a Proto-Gravemind). This form is unlike most other Flood forms since it lacks any sort of mobility or defense, and is passive. These are, theoretically, the beginning stages of a Gravemind form. It is believed to be the product of syntactical fusion of several combat forms, merging its entire subordinate forms into a massive, bloated spherical organism with spindle-like outgrowths that adhere to the surrounding environment.

Proto-Gravemind forms are unique in that the consciousness of the original host organisms are still vaguely maintained, greatly unlike most combat forms, allowing for slight manifestations of mental resistance to Flood assimilation and mental communication with other humans. However, the Flood is able to, with high intuitive perception, sift through the individual memories of the original hosts, deleting erroneous ones while seeking memories that are of great importance to the overall Flood species. Once such a memory is found, it is incorporated into the general consciousness of the Flood species.

The only known human example currently is Captain Jacob Keyes, who was originally assimilated into a Combat Form, and was then merged with the Proto-Gravemind on the bridge of the CCS-class battle cruiser, the Truth and Reconciliation. He was transformed into a Proto-Gravemind as he had superior knowledge about the Pillar of Autumn and Cortana. It was destroyed when a squad of Special Operations Elites infiltrated the bridge after John-117 had escaped the cruiser aboard a Covenant Banshee.

Gravemind
"I have beaten fleets of thousands! Consumed a galaxy of flesh and mind and bone!"

The Gravemind is the leader intelligence of the Flood. There can only be one at a time, although if one is killed another may form, though it is unknown if the dead Graveminds knowledge is passed on to the new one. The statement above supports this, but it has not been confirmed by Bungie. Another thing that may happen is that the Gravemind is the same from before in a different body. The Gravemind is the assumed highest state of transformation that a Flood form can achieve. It is believed that once a "critical mass" of Flood forms has been synthetically fused, similar to the Brain Form, but on a larger scale (and very likely the next stage of a brain form), a Gravemind form is created. It assumes a centralized sentient intelligence that is believed to lead the general Flood species itself through unknown means, possibly telepathic. It also appears to hold all of the Flood's memories, even those of individual organisms, and of the previous Gravemind as well, through extraordinary and enigmatic "genetic" means. While it is capable of directing the overall Flood species, it also possesses micro-managing capabilities for small encounters.

The Gravemind is the personification of Flood psychology, ravenously seeking to assimilate all sentient life in the galaxy, and is of unusual intelligence: of inventive plans for Flood propagation and serpentine delusions and betrayals, capable of manipulating others to do its bidding. It appears that only one is present at a time : one existed over 100,000 years ago, but was destroyed by the first firing of the Halos. Another was recreated afterward, and settled in the depths of Installation 05: it showed remarkable understanding of Forerunner technology, even going so far as to capture the resident Monitor, 2401 Penitent Tangent, manipulating the installation's teleportation system, and understanding the purpose and intricacies of the Halo installations. The Gravemind is also remarkably long-lived, surviving for a millennium.

Physically, it somewhat resembles an engorged Venus Fly Trap: it is a "massive, tentacled...entity" with a gigantic, fleshy neck and indistinct head, prominently featuring a massive mouth that speaks in low, gravely tones. It possesses massive tentacles of extraordinary length and strength, capable of stretching from the dark interior of Installation 05 and believed to be extending hundreds of kilometers to above the surface of the Halo. It is also believed that a Gravemind can weigh several thousand tons.

Flood Swarms
The Flood Swarms are the first seen flying incarnation of the Flood. They travel in large airborne packs and emit barb-like projectiles to puncture enemies similar to the Ranged Pure Form.

Flood Bomber Form
Another airborne parasite, these large floating creatures move slowly and have the ability to drop Flood Growth Pods full of Infection forms on enemies. This appears to be their only form of attack, making them very vulnerable when attacked from a long distance. The Bomber's role of dropping Infection forms on enemies is similar to the Carrier Form but instead of blowing itself up at close range, it drops pods filled with Infection forms from the air, thus making it more useful at spreading infections than the Carrier Form in most situations.

Thrasher Form
The Flood Thrasher Form is a Flood form, similar in purpose and behaviour to the Flood Tank Form, but larger and somewhat resembles a Thorn Beast. It is intended to quickly close with enemy ground units, using a devastating melee attack to deal significant damage. So far, they have only been witnessed as a part of the Flood forces on the Flood-controlled Shield World. These have only been seen in Halo Wars.

Building Forms
The Flood Barracks (called "Flood Den" in the final version) are organic structures that ceaselessly churn out a variety of Flood forms and are featured in Halo Wars.
 * Flood Barracks

They can't be destroyed by "conventional" weaponry, but players can damage them enough to force them into a withered, dormant state. The effect is only temporary, and given enough time, the Barracks will recover and begin producing more Flood forms. It is speculated that the Flood Barracks will only produce Pure Forms as it is a natural "building" and Combat Forms require a host to be created, although it has been stated that the flood have not achieved the "Pure Form level" at this time, as the Gravemind has not been fully spawned yet, and if that's the case, it probably "stores" infection forms, and constructs biomass, since on the picture shown, the flood biomass seems to be built around a Covenant Citadel, likely having plenty of hosts to send into battle.

The Flood Root resembles a large tentacle capable of "whipping" units that get close to it. It is perhaps the equivalent to a defensive Turret, although it is unable to attack air units and its effective range seems quite short. When attacking, it will swipe and slam enemy infantry. In some cases it will grab a unit and lift it into the air, after which it actually dissolves into the same matter that a Flood spore cloud is made of.
 * Flood Root

They are seen in more heavily infested areas, and are attached to the Proto-Gravemind. When destroyed, the main building form becomes weaker. In the Halo Wars Campaign, the Colony Form cannot be destroyed by normal means, and instead can only be reduced to a dormant state which it will soon recover from. On multiplayer maps featuring the Flood, however, Colony Forms can be permanently killed.
 * Colony Form

It can create Infection Forms and Carrier Forms. The Nest has a number of tentacles surrounding an opening which resembles a set of lips, which spits out swarms of Flood forms
 * Flood Nest

It fires some form of flood-goo out of the hole at the top of the structure at enemy forces. In the level Shield World from Halo Wars, they are used as an anti-air unit, capable of shooting down the evacuation pelicans for Alpha, Bravo and Charlie Platoon. Unlike other Flood building forms in Halo Wars, it can be killed completely by conventional means, instead of going into a dormant state from which it will soon recover, in both Campaign and Multiplayer. It looks very similar to the Unnamed Organic Tower-Like Form perhaps they are the same Building. Occasionally, the Flood Launcher's ordnance will release a cloud of Flood spores.
 * Flood Launcher

The Flood Den is a medium-sized building form which Flood Swarms and Flood Thrasher Forms come out of. Like all Flood building forms in Halo Wars, it cannot be killed by conventional means, instead going into a dormant state from which it will eventually recover.
 * Flood Den

The Flood Vent is a large vent covered in Flood biomass which creates Flood Bomber, Infection and swarm Forms. Like all Flood building forms in Halo Wars, it cannot be killed by conventional means, instead going into a dormant state from which it will eventually recover.
 * Flood Vent

Flood Stalks are a structure that if destroyed, releases a couple of groups of infection forms. It seems to consist of a few of Flood Growth Pods on a biomass stalk.
 * Flood Stalks

Shielded Carrier Form
Shielded Flood Carriers are a variant of Flood Carrier Forms with what seem to be Jackal Point Defense Gauntlets integrated into thier midsection.

Flood Juggernaut
The Flood Juggernaut is the name of a large Flood form (possibly pure) found encoded in the Halo 2 disc, but is not actually seen in gameplay. Named for its size and power, the Flood Juggernaut is very large, about twice as tall as a SPARTAN II or Sangheili, and its mighty tentacles can kill most enemies in a single strike. The Juggernaut was created for Quarantine Zone as a means to stop the Enforcers, and as one can see, that would have been an easy task for the Juggernaut, thanks to its size. The Flood Juggernaut can leap twice as far as the already impressive jumping capability of a Combat Form, and it has the power of a Tank Form. It is also extremely durable, as seen during a battle where a pair of Juggernauts were modded into the level High Charity. It was not in the final version of Halo 2, and it is only accessible on High Charity through modding. The Juggernaut is believed to have been cut due to its being overpowered, as it was able to kill John-117 in a single strike. Most speculate that it was an early version of the Flood Tank Form, as it bears many physical similarities to the form. They are believed to have been cut at a late stage of development, due to the fact that it's model and code were almost complete.

Infector Forms
The Infector is a three-limbed form that features the ability to regenerate destroyed limbs within ten seconds, making it a formidable foe. Their body is very similar in appearances to the Carrier Form, except that the Infector Form is designed to shoot out spores, not explode and release infection forms. They are only seen in the Art of Halo 3.

Transport Forms
The Flood Transport Form was a Flood Purestrain form. Unlike the Carrier Form, the Transport carries up to six non-armed Combat Forms. It had four triple-jointed walking arms to move its load of troops around, and featured numerous tentacles to consume or recycle bio-mass. It is only seen in the Art of Halo 3.

Stealth Forms
Like the Stalker Form, it excelled in the art of stealth to defeat the enemy and is the only Pure Form to be able to use weapons with the aid of two four-tentacled hands. It is only seen in the Art of Halo 3

Pod Forms
Flood Growth Pods and Flood Dispersal Pod are biomass created forms for flood unit deploy in the battlefield. Flood Growth Pods are large, spherical collections of biomass that grow on the surfaces of ships, buildings, and other structures. They are started when mass Flood forms enter an area, or when Flood Spores enter the area. They incubate developing Infection Forms. They are only seen during stages of advanced Flood infestation when a Gravemind has formed and consumed enough flesh, to support Pure Forms. They have no forms of attacks, but if they are attacked, they explode and release their infectious spores and their Infection Forms. They tend to release about 25 Infection Forms if shot at or if hit by the blast from an explosion in their immediate vicinity.

They are first seen in Halo 2, during the Flood Invasion of High Charity, though it was only at development stage. Later in Halo 3, it was encountered more in the Campaign levels Floodgate and Cortana, and in the multiplayer maps Isolation and Cold Storage, though the multiplayer pods do not release infection forms.

The use of Flood Growth Pods, is quite similar to Carrier Form Flood, they incubate the Infection Forms for a certain time and if the pods are agitated (i.e. being shot) they burst open and spew Infection Forms (between 5 and 20 Infection Forms). Unlike the Carrier, however, the pods are completely idle and mostly defenseless. They can't suicide bomb the player like a Carrier can, but still pose a threat due to the large number of Infection Forms they release. In Halo Wars, the Flood Growth Pods now serve as double-edged swords: shooting them will sometimes releases precious resources for weapon and vehicle upgrades. But other times, they will release either a swarm of Infection Forms or a cloud of flood spores, both of which can destroy infantry squads in seconds.

The Flood Dispersal Pod is a Flood dissemination mechanism seen only in Halo 3 and rarely in Halo Wars on the level Cleansing (level).

In essence, and in practice, the Flood Dispersion Pod is the equivalent of the HEV drop pods used by UNSC ODSTs and Orbital Insertion Pod used by Sangheili Special Forces. Rather than encasing its occupant(s) in protective Titanium-A and lead, the dispersal pod uses Flood biomass to shape the pod and protect the occupant from the stresses of orbital deployment.

Large dispersal Pods were deployed by High Charity as the Gravemind arrived with its army at Installation 00, scattering Flood forces all over the artificial world, delivering significant numbers of Combat Forms to the surface. It is presumed that the Flood infestation was spread across the unfinished Installation 04 (II). It is also unlikely that the pods have a navigation system. Later, after the destruction of High Charity, more pods were used to deliver Combat Forms to the Control Room of the new Halo built by the Ark, in an attempt by the Gravemind to stop the Halo's activation. They are similar to the Infection Forms in appearance, but obviously not in size.

Combat Behavior
While possessing an unusual sentient intelligence, with Combat Forms still retaining extremely high-level cognitive functions such as vehicular operation, lower level cognitive abilities, precise dexterity, and overall battlefield tactics, are notably lacking in the Flood, especially in their psychological and combat behavior. While the Gravemind coordinates Flood strategy on a galactic scale, small encounters are not coordinated between individual Flood warriors.

The Flood follow a zealous and ravenous strategy with little target selectivity, aggressively attacking all hostiles in range. Furthermore, Flood tactics are based primarily on asymmetrical warfare: the Flood uses all types of its different forms to kill the enemies that it sees. Usually the Infection, Combat, Carrier, and Tank Forms rush the enemy and attempt to quickly eliminate them. The RangeD Forms try to support their zealous comrades with ranged attacks. The Stalker Forms attempt to transform into a Tank or Ranged Form. The Combat Forms also utilize any weapon that they have picked up. While this strategy is observed to be a Zerg Rush, they aren't completely suicidal, since they usually dodge dangerous ordnance.

The Flood have a strong preference for close-range combat: combat forms possess abnormally strong melee attacks by whipping their tentacles, capable of dropping a SPARTAN-II MJOLNIR Armor’s shields in half with a single hit. Carrier forms cause tremendous devastation at close ranges, and their explosions are extremely devastating to friends and foes alike at close range, with the added benefit of disseminating infection forms in the immediate vicinity of enemies. Infection forms are capable of attacking only at point-blank ranges, exploding in damaging bursts when meeting an opponent's personal energy shields or instantly grabbing hold of an unshielded enemy organism and infecting it. Combat forms can also use firearms, and have been seen clutching them in one of their hands (regardless of whether it is one-handed or two-handed). However, combat forms are extremely poor warriors at range with ranged weaponry: it is believed that the neurological ramifications of Flood infection significantly decrease accuracy, as well as weapon recoil if held in only one hand. The Flood are mostly only dangerous with heavy weaponry, such as the M19 Rocket Launcher or the M90 Shotgun, and are curiously ill skilled with close-quarters weapons such as the Energy Sword. However, it is still possible that a Flood with a Energy Sword can kill a fully shielded Elite or Spartan in one blow.

However, as mentioned previously, the Flood demonstrates a curious higher intelligence despite their poor combat aptitude and coordination. The Flood collectively learn any general information a species possess, even if only one member of that species is assimilated. For example, during the ill-fated raid on the Infinite Succor, after even one Sangheili was assimilated, all the Flood could open any doors and access any system on the ship immediately afterward, demonstrating that the Flood can share collective knowledge across their species. Furthermore, when the Flood assimilated the Prophet Legate, the Flood reflected the high-level knowledge of the Legate through its individual organisms to Special Operations Commander Rtas 'Vadumee, mocking him with the words, "the Forerunner could not defeat us...what chance you?". The Flood, however, reflect unusual knowledge in the individual psychologies of its individual species in processes facilitating increased infestation: The Flood were sighted gathering bodies in the corners of the Covenant ship Truth and Reconciliation, perhaps making it easier for Infection Forms to find hosts to reanimate. The Flood were also gathering body parts for a new Brain Form aboard the Infinite Succor. Flood Combat forms have been sighted crudely operating Ghosts, Warthogs, Wraiths, and Scorpions in ground operations, although with slow deliberation and clumsy driving and accuracy. The assimilated crewmen of the UNSC Frigate In Amber Clad were also able to initiate a Slipspace micro-jump for the frigate and perform rudimentary maneuvering of the In Amber Clad, navigating the frigate from the atmosphere of Delta Halo to High Charity and then crashing the vessel into a tower, releasing Pelican dropships to rain down upon the capital.

The Flood's millennia of existence are believed to have made the assimilation process unusually brief: creatures killed in a short firefight can be reanimated within seconds to minutes by an infection form, and live creatures can be mutated into fully grown and mentally devious combat forms within minutes of contact. The mutations can occur randomly, or uniformly: according to the Halo Graphic Novel, they can sometimes develop into engorged combat forms notably larger than the normal combat form, or can alarmingly grow thick shells within minutes, indicating that the Flood can facilitate tremendous and exaggerated metabolic activity during the physiological mutilation. Furthermore, in rare cases, the mutations can constantly shift: when Rtas 'Vadumee battled his comrade-turned-combat form Bero 'Kusovai aboard the Infinite Succor, 'Kusovai continued to mutate throughout the duration of the battle. Though never seen, it can be assumed infection forms were involved in infecting victims in this case. Before being killed to prevent his becoming a Flood, one of the Sangheilis accompanying Rtas 'Vadumee appears to have a wound in his chest that may have been the entry point for an infection form. Many of the Flood on the Succor seemed to constantly be mutating the infected biomass and also seemed to merge them together. This suggests the possibility that over time any infected host can mutate further and further, as exemplified by the Brain Form or the Gravemind and different versions of the Combat Form.

Known Victims

 * Minister of Etiology
 * Wallace A. Jenkins
 * Captain Jacob Keyes
 * Sub-Commander Bero 'Kusovai
 * Private Manuel Mendoza


 * Prophet of Mercy
 * Private Riley
 * Prophet of Regret
 * Prophet of Truth

Halo: Combat Evolved
It is best to use the MA5B Assault Rifle, Shotgun, or M6D Pistol against the Flood. The pistol is capable of killing a combat form with a single shot to the chest, but is generally not recommended for mass engagement, as its medium rate of fire and small magazine size can lead the player into a position where they are surrounded by Combat Forms, therefore, it is best to only apply it in long-range combat. Furthermore, Combat Forms have a tendency to revive after being shot once with a pistol. The Plasma Rifle is also a good weapon of choice to use (if the assault rifle/shotgun are unavailable) as its high rate of fire can quickly destroy infection forms while still being potent enough to destroy the combat forms.

The Sniper Rifle is ineffective against the combat forms and will not kill them. The Shotgun on the other hand is the best choice against Flood Forms, as it usually finishes them off in 1 or 2 shots, especially Combat or Carrier Forms. On lower difficulties, Shotgun ammo is almost always available.

The Assault Rifle, an all-round effective weapon with large spread range, is superb against masses of combat forms and infection forms. Never bother to melee Combat Forms or Carrier Forms, as it is practically useless and very time consuming. For infection forms however, a single melee can take down four to five of them.

All in all, the most practical option is to have a shotgun for close-up/quick work, and a Pistol or Assault Rifle for longer range. The pistol is most useful for the level Two Betrayals, while the Assault Rifle/Plasma Rifle is preferred on The Library due to the levels cramped nature.

It is possible to shoot the arms off of Combat Forms (alive or dead). Dismembered Combat Forms pose no threat. This works well on the level Keyes

Halo 2
Dual wielding SMGs and other dual-wielding weapons has proven to be very effective, as well as the Sentinel Beam, Shotgun and Energy Sword. Plasma Pistols, however, are the least useful. Dual-wielding Plasma Rifles, and aiming at the arms of the Flood is more than enough to rip them apart easily. The arms will come off quickly and therefore leave the combat form, quite literally, unarmed. They will waddle about and explode after a few seconds. There is also a way of killing combat forms in one shot. On the chest, you can see the infection form's tentacles waving about. This is where the infection form has burrowed. Shoot the infection form, and unless the combat form is utilizing shields, the combat form will die. The SMG is also an excellent weapon against the Flood, as long as ammo is in large supply. Also effective is the BR55 Battle Rifle, as aiming for the Infection Form in the chest of a Combat Form will kill the Infection Form controlling it in a single burst. It is often a good idea to use the Brute Shot's melee, as it will have the same effect as an Energy sword on Combat Forms; it will destroy the body and prevent Infection Forms from reanimating it. For one of the best combos, use Dual SMGs for Combat and Infection Forms, and keep a Shotgun in reserve. Another highly effective combo is to carry a Battle Rifle for engaging the Flood at long ranges and carrying a Shotgun for when you don't have enough time to be precise with your aim. A good strategy on The Oracle is to dual wield a pair of Needlers, you can usually do this without running out of ammo. Also, don't let the infection forms touch you if you have no shields; otherwise, they will kill you instantly.

Halo 3
The Incendiary Grenade instantly destroys all pure Flood forms and is the one true weakness of all pure Flood forms, as well as its Support Weapon counterpart the Flamethrower. Also, all forms can be instantly destroyed with a melee, with the exception of the Pure Forms, which cannot be revived anyway when killed. There is also a way of killing combat forms in one shot. On the chest, you can see the infection form's tentacles waving about. This is where the infection form has burrowed. Shoot the infection form, and unless the combat form is utilizing shields, the combat form will die. However, the corpse will be susceptible to infection by another infection form unless the body is in fragments.

Infection Forms
Either shoot them with an automatic weapon, such as the Assault Rifle or Plasma Rifle, or waste groups of them with explosive weapons, such as the Brute Shot, Frag Grenade, or the Incendiary Grenade. One of the most effective weapons against infection forms is the Sentinel Beam with it's ability to destroy dozens of infection forms with a few sweeps. With their low health and attack, the only saving grace of these pathetic units is their vast quantity and their ability to instantly kill a player with their shields down.

Combat Forms
Human Combat Form/Brute Combat Form The Human and Jiralhanae combat forms are, despite the anatomy of the unit, the same in terms of attack and damage. With a high resistance to automatic weapons, a prime choice against the two Flood forms would be the Battle Rifle, due to its high rate of headshots, the only weakness of the Flood form, or its Covenant counterpart, the Covenant Carbine. The Magnum pistol is also a good choice due to it's ability to knock them onto the ground. Make sure you bash it when it is on the ground or it can get re-infected. Using sticking grenades such as Plasma Grenades and Spike Grenades also dispose of these enemies quite quickly. In close-combat, the melee of most weapons, a single shot from the Mauler or the famous Energy Sword, Gravity Hammer, or Shotgun will kill them quickly. Despite this, they are famous for the ability to turn all weapons, from SMGs to Rocket Launchers, into deadly weapons, as well as their hard melees.

Elite Combat Form

The exact same as the Human and Brute forms, although the Elite combat form sometimes uses the energy shielding of its former host to its advantage. On higher difficulties, such as Heroic, the Plasma Pistol and Plasma Rifle serve excellently in lowering their shields.

Pure Forms
Stalker Forms

The weakest of the forms, although ironically it has high resistance to all types of weapons, excluding the Energy Sword and Gravity Hammer, which kill most, if not all Pure Forms in one shot. They seldom fight, although their quick melees are not to be underestimated. It is suggested to kill them in these forms before they mutate.

Tank Forms

Argued to be the strongest of the Pure Forms, along with the Ranged Forms, the Tank Form has high resistance to all weapons from all parts of its body except for its mouth. An automatic weapon full burst to its mouth or an Energy Sword will easily dispose of the Tank Form. A Shotgun serves your needs just as well, although its melee is to be feared. Of course, Fuel Rod Guns, Rocket Launchers, and the Brute Shot's melee are pretty effective against them.

Ranged Forms

Also argued to be the strongest of the Pure Forms, as well as the most annoying enemy in the entire Halo franchise. They attack from long distances and in rapid, accurate bursts, making counter-sniping difficult. The Sniper Rifle and Beam Rifle are viable weapons to exploit, but otherwise it is a recommended tactic to close-in on the Ranged Form and cause it to mutate. A warning to counter-snipers is that it will occasionally guard its vulnerable mouth when shot upon, making it resistant to all forms of weapons. Short bursts from the Assault rifle or plasma rifle, or a carbine. Brute Shots can also prove to be effective against them.



Halo Wars


Their tactics on Halo Wars are very similar to those they perform on the Halo Trilogy: they can obviously infect Humans, Elites, Brutes and now even Jackals and Grunts with their Infection Forms, and usually attempt direct attacks with large groups. They possess large tentacle-like Flood forms and large pods that can pin in and attack vehicles, in the case of the Elephants they capture, which is an in-game objective to rescue in the first level where are encountered in-game. The Flood establish bases by capturing and spreading an infestation on enemy bases.

The Flood have been confirmed by Ensemble to have been made overpowering to both UNSC and Covenant forces to keep their appearance terrifying. Early in development, Ensemble considered making the Flood a playable race, but this idea never made it past the concept stage. According to lead designer Graeme Devine, the main issue was that in order to balance the Flood with the UNSC and Covenant, the Flood would have to have been similar to StarCraft's Zerg, which did not mesh with the Flood's role in the games, to be "the single scariest thing in the galaxy."

Several new Flood forms and recurring forms with new functions appear:
 * Ground Forms
 * Infection Forms have the ability to crawl over walls, increasing their potent abilities, and are able to reach ground no other forces can reach. When a group reaches an enemy soldier, they will attack them, and slowly infect them, turning them into Combat Forms. Try to avoid using infantry when facing them, but fight them with vehicles.
 * Combat Forms are the standard fighting force of the Flood. Human, Elite, Jackal, Grunt, and Brute forms have been seen. Hellbringers are the most effective unit to take them out.
 * Carrier Forms are a suicidal unit which explodes and delivers its payload of Infection Forms directly into enemy lines. Kill it when it's still far from your troops.
 * Thrasher Forms quickly close in on enemy ground units, using a devastating melee attack to deal significant damage. Contrast them with infantry.
 * Flood Growth Pods now function as a double-edged sword: shooting them will sometimes releases precious resources for weapon and vehicle upgrades. But other times, they will release a swarm of Infection Forms that can destroy your squads in seconds.
 * Air Forms
 * Flood Swarms are the first-ever-seen flying incarnation of the Flood, and were described by developers as "infected buzzards crossed with evil bats". These creatures travel in teeming airborne packs, and can emit barb-like projectiles to puncture enemies. Wolverines are most effective against them.
 * Bomber Forms drop Flood growth sacs full of infection forms on targets.
 * Building Forms
 * Flood Barracks are organic structures that ceaselessly churn out a variety of attackers. They can't be destroyed by "conventional" weaponry, but players can damage them enough to force them into a withered, dormant state. The effect is only temporary, and given enough time, the Barracks will recover and begin producing more Flood forms.

Changes from Halo: Combat Evolved to Halo 2

 * The Flood can make screeches and roars, which were not present in the first game.
 * Skin is more textured, giving the Flood a more realistic and disgusting look. The faces of the Human forms are also more readily recognizable, an artistic touch which serves to remind players of the threat the Flood pose to Humanity.
 * If a Combat Form has both arms shot off, it will walk slowly then explode, releasing an Infection Form. This eliminated the Flood Buddy glitch.
 * Rather than having to "gather bodies," the Flood can seemingly infect hosts over the course of a few minutes or even a few seconds.
 * Rather than corpses re-animating themselves, an Infection Form will enter the body, then re-animate it.
 * If a player is jumped by an Infection Form while their shields are down, they will die instantly. In Halo: Combat Evolved, it would simply take a bar of health away.
 * Some Combat Forms derived from Elites still have functional energy shields.
 * The Gravemind is introduced, making the Flood a more significant faction in the series.
 * The Flood can use vehicles, albeit clumsily.
 * The melee attack does little to no damage against the Flood in Halo: Combat Evolved. It does a bit more damage in Halo 2.
 * The Flood appears to be "smarter" when in combat, preferring to stay back and fire on the player rather then "rush" him/her.
 * In Halo: Combat Evolved a Combat Form would walk slowly when shooting at the player. When losing the arm that shoots, it would go "berserk": charging at the player, madly flailing its arms. In Halo 2 the combat forms can "decide" whether to use a melee attack or to shoot at the player.
 * If a player is wielding an Energy Sword or Shotgun, a Combat Form may retreat. They will not do this if a player is wielding other weapons.
 * A Juggernaut Form was put in, but was not placed in any accessible area of the game, although it was kept on the disc.
 * Unlike Halo: Combat Evolved, shooting one Infection form will not result in the rest of the swarm exploding with it, although there is a significant decrease in their number.
 * Elite Combat Forms do not have the active camouflage anymore.
 * Flood Combat Forms do not "feign death" anymore.
 * Flood Combat Forms can board your vehicle, and then will proceed to melee the player.

Changes from Halo 2 to Halo 3

 * Brutes may now be infected and transformed into Combat Forms.
 * Melee attacks and plasma weapons were nearly useless in Halo: Combat Evolved and Halo 2, but now can be used effectively against the Flood.
 * Also, any weapon can be used to dismember Combat Forms, either by melee or shooting. A realistic touch, considering the Flood's weak, rotten flesh. Thus, all Combat Forms are significantly easier to kill.
 * Infection Forms can mutate humans, unshielded Elites and Brutes into Combat Forms in a matter of seconds.
 * Pure forms are introduced, consisting of the Tank Form, the Stalker Form, and the Ranged Form. In addition, the Flood Growth Pods and Flood Dispersal Pods can be seen in the eighth Campaign mission, "Cortana."
 * Gravemind has the ability to speak through his minions during combat.
 * Combat Forms wield weapons far less than in both Halo: Combat Evolved and Halo 2.
 * Carrier Forms release significantly more Infection Forms when they explode. The explosion does not push the player away much as in Halo 1 and 2. In addition, Carrier Forms are significantly more difficult to kill with projectile weapons, but they are easily destroyed by Plasma Weapons.
 * If an Infection Form hits you while your shield is down it will not only make a crunching noise, but also it will most likely kill you on higher difficulties.
 * In the level The Covenant, the Flood helps you on your way to the Prophet of Truth, but then turns against you once the Arbiter kills Truth.
 * Running over Infection Forms in a vehicle can result in a bumpy ride. The Infection Form that was hit by the vehicle will not pop.
 * Carrier forms will blow up and no infection forms will come out if it is hit by an incendiary weapon, a Plasma Grenade, or Spike Grenade.

Changes from Halo 3 to Halo Wars

 * In Elite forms at least, Flood Combat Forms no longer push back the heads of their victims. In fact, the Elite's head now moves with the form as if the Flood actually use it now. In addition, Flood Combat Forms have more tentacles than before, and resemble spikes rather than flexible appendages.
 * Infection forms are somewhat "chubbier" if compared to the trilogy version.
 * Grunts and Jackals are now able to be converted to Combat forms.
 * The Infection Forms don't "explode" when they are shot in the cinematics.
 * There are many new forms of Flood introduced in the campaign.

Trivia

 * The only game the Flood is absent from is Halo 3: ODST.
 * Part of the Flood's Binomial nomenclature, Inferi, is Latin for "The Dead", and could be a reference to Roman mythology, as this was their name for the Netherworld and it's inhabitants. Inferi means 'The Dead' and Redivivus means 'renovated, reconstructed, rebuilt or recycled' so literally Inferi redivivus means 'The Dead Reconstructed'.
 * The Flood bear a strong visual resemblance to an enemy cut from Marathon 2, the “fungal zombie.” They were to be the result of a bioweapon invented in the final days of the ancient S'pht and reused by Robert Blake's forces on Lh'owon, designed specifically to target Pfhor. Both the fungal zombies and Flood were principally designed by Robert McLees.
 * There is a species of fungi that is frighteningly similar to the Flood, the cordyceps unilateralis. They attack carpenter ants and take over their neural system in order to make the ants go to high places. By the time it gets there, it's been rotted from the inside out like a Flood host and it releases spores from that vantage point.
 * In a PC World article, the Flood was voted as #31 of the top 47 "most diabolical video game villains of all time."
 * The Flood was inspired by Christopher Rowley's The Vang.
 * In Halo: The Flood, Master Chief was attacked by an infection form while his shields were low. The infection form had slit the under parts of his armour, exposing his neck, and almost tapped his spinal cord. If not for the quick use of an electrical shock by Cortana to kill the form, Spartan-117 would have been infected like many before him.
 * The Flood are similar to Tyranids of the Warhammer 40,000 tabletop game and alien Xenomorph of the series of the same name.
 * The name "the Flood" is a biblical reference to the Great Flood in which God destroyed all life except for that which was carried by Noah's Ark.
 * Sgt. Avery Johnson was the first known human to survive a Flood infection attempt.
 * The thick ooze-like blood of the Flood in Halo 3 has a striking resemblance to a real world substance known as Fuligo septica,(also known as, "Dog Vomit Slime Mold") which shares the same thick phlegm-like characteristics as the fan named, "Flood Blood".
 * The Flood bear a major resemblance to the Many of the game System Shock 2. Similarities include their ability to control other organisms via a parasite, a hive mind based around a central intelligence, controlled organisms appear to be dead, and the fact they can assimilate the intelligence and memories of whatever they infect.