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" and designated LF.Xx.3273 by the Forerunners), or The Parasite as they are known to the Covenant, are a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient life in the galaxy, including the Forerunners, during the 300-year-long Forerunner-Flood War. The Flood presents the most variable faction in the trilogy, as it can infect and mutate Humans and Covenant species, such as Elites, and Brutes, into Combat Forms. They are widely considered to be the greatest threat to the existence of life, or, more accurately, biodiversity, in the Milky Way Galaxy.

Background
Originating from an unknown world outside the Milky Way Galaxy, the Flood is an extremely adaptable, prolific and dangerous parasitic life form. The species grows and spreads by infecting other organisms, hijacking their bodies and nervous systems in order to transform them into one of many specialized Flood forms. Through infecting sentient life-forms, the Flood assimilates their memories and intelligence, and the species becomes collectively more intelligent as a result. Because of their frighteningly rapid growth rate, as long as enough hosts of sufficient biomass and intelligence are available, the Flood is effectively unstoppable.

They are the cause of the "Forerunner-Flood War", which lasted approximately 300 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 other strategic option, the Halo Array was fired, resulting in the elimination of the original source of the Flood infestation and the death of almost every sentient being in the Galaxy, save for several beings placed on the Ark by the Forerunners.

The Flood exists in a fairly stable state in the absence of a Gravemind. They lack the ability to spawn spontaneously and spread by infecting 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.

A Gravemind is the guiding intelligence of the Flood, capable of controlling all Flood forms across vast distances. With the formation of a Gravemind, the assimilated intelligence and memories of consumed hosts become part of a collective consciousness, which is capable of strategic thinking and commanding individual Flood forms in an organized manner. The control it exerts over the more mobile forms it creates is accurately likened to an entity's control over its own limbs.

Although little 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 more or less identical 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 a "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".

Origins
The Flood are believed to have arrived in the Milky Way Galaxy from an unknown galaxy, more than 110,000 years prior to 2552. They were first discovered by humans near the edges of the Milky Way, aboard automated vessels of unknown origin which had supposedly arrived from one of the Magellanic Clouds near the Galaxy. These vessels, which were found crashed on both inhabited and deserted planets, contained millions of containers filled with a fine organic powder (presumably Flood spores or even simpler Flood biomatter). Humanity studied the material and deemed it to be harmless, but noted that it had psychotropic effects on lower animals. Small dosages were administered to Pheru, popular domestic animals among humans and San'Shyuum, and resulted in more docile behavior. Unbeknownst to the humans or San'Shyuum at the time, the powder was altering the genetic code of the Pheru, manifesting as improved behavior while slowly changing other aspects of their genome.

After several centuries, more disturbing changes appeared. Pheru treated with the powder began exhibiting furry growths and fleshy protrusions, which other Pheru were compelled to consume. Shortly afterward, the Pheru began suffering widespread abortions and genetic deformities, and were killed or released into the wild. It was at this point that humans and San'Shyuum began to feel the effects of the "disease": infected victims began engaging in cannibalism of other individuals, thus spreading the infection. Only then did the Flood truly reveal its nature, as it began reshaping victims into horrific forms and taking over hundreds of worlds. The growing infestation led to the Human-Flood War.

Remarkably, during this time humans discovered a cure for the Flood, and managed to wipe it out while the infection was still comparatively minor. The remaining Flood-carrying ships escaped the galaxy, and the Flood was never to be seen again until the incident at G617 g1. During the interim, humanity (who was then at war with the Forerunners) destroyed all of the original carrier vessels that they had discovered. In addition, no trace of the Flood remained among the Pheru, humans or San'Shyuum.

Because of the lack of evidence, most Forerunners were unaware of the existence or true nature of the Flood, and many dismissed it as a simple excuse for humanity's aggressive expansion. However, several high-ranking Forerunners were fully informed of the threat, and in secret began developing countermeasures should the Flood ever return.

Forerunner-Flood War
"It fed on intelligent life and in doing so, became ever more intelligent itself. The Flood was unique, it used their very strength against them."

- Cortana on the Forerunner-Flood War.

The Flood reappeared as a viable threat in the galaxy when they first encountered the Forerunners on the planet G617 g1. Initially, the Forerunners severely underestimated the potential threat this new life form posed, and used tactics more suited to disease outbreak than actual warfare. By the time the infection had begun to spread, the Flood had become a much deadlier and more intelligent foe. 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. The sheer numbers of Flood forms on the planets overcame Forerunner ground forces. 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 an Artifact to generate a slipspace portal leading to the Ark on Earth and possibly other uninfected home worlds of Tier 7 races they considered "worthy", such as the various species that would make up the Covenant.

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 the Halo Array or protected by the Shield Worlds, which were also out of range.

The Flood was contained, and unable to reproduce and grow 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. Containment measures on this planet had failed catastrophically, and the entire surface of the planet was choked with Flood biomass. Ground forces were deployed to investigate the presence of unknown hostiles sighted by the ship's radar. After fighting their way through Covenant forces, Sergeant Forge and several Marine squads were attacked by the Flood.

The surviving UNSC forces, which included Sergeant Forge, were ordered to recover some Elephants 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 assembled a larger force to rescue the other soldiers. Following their successful rescue, the UNSC forces prepared to engage 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 was wiped off by the ship's crew, Sentinels, and the Shield World's cleansing rings. All the Flood forms 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
"The only way to stop the Flood is to starve them to death."

- Cortana to John 117 and 343 Guilty Spark.

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 crewmembers 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 them to reproduce by spores.

The few surviving Covenant soldiers left in the facility 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" (First Staff Sergeant Marvin Mobuto, then 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 ODST 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 Antonio 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.

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 the Arbiter 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 the special operations team could do about it.



During their pursuit of Sesa 'Refumee, Thel 'Vadamee and Rtas 'Vadumee, along with Spec Ops Elites and Grunts, entered a laboratory, while 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 Elites, Heretic Grunts, 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 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 Sangheili 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 Elites 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 attack, led by Thel 'Vadamee and involving a small team of Spec Ops Elites, using a gondola to get to the index while attempting to control the gondola and keep it from slipping into Flood or Sentinel hands. Later, the In Amber Clad was captured by the Flood.

Attack on High Charity
Spartan-117, who had arrived with Keyes and Johnson, met Thel 'Vadamee 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


At the conclusion of the Battle of Voi, with the Prophet of Truth activating the Artifact 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 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 first destroying High Charity's reactors, causing a massive explosion and the destruction of the wrecked city, then activating the unfinished Halo using the Index Cortana retained from the first Halo. The ring's activation also presumably killed the Gravemind.

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 not all of the Flood on Installation 05 left and there are still some there, since Installation 05 was never destroyed), or any other remaining Forerunner Installations. The fact that the Flood are not originally from this galaxy further complicates the issue, and how and where they originate from leads to more possibilities. It may explain 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.

Biology


The Flood is an anomaly in all known biology. Their mode of growth and reproduction makes them incompatible with any natural ecosystem. They are extremely adaptive, and mutate hosts to suit the species' current needs.

All Flood biomatter is made of a unique, undifferentiated "Super-Cell", which relays information and coordinates the cellular assembly like traditional neural cells. It is also capable of flexing and moving like muscles, and can be arranged to mimic any bodily organ that is required for the Flood's use. This combination of traits has earned it the analogy of "thinking muscle", due to its versatile nature.

The Flood Super-Cell physically resembles both neuron and glial cells in structure, with a central cell body and many tendril-like and branching structures.

In order to develop beyond the most basic stage, the Flood needs to infest and assimilate other species, most significantly sentient life forms. This is a multi-step process, beginning with an Infection Form attaching itself to a host, hacking into its nervous system, paralyzing it, and forcing a resonance frequency with the host's nervous system. To begin mutation of a host, Infection Forms inject encapsulated Flood cells into the victim's body.

The cells attach themselves to the target cells and insert their DNA into their new destination, which in turn transform the infected cell into another Flood cell. This process of cellular attachment and genetic transfer occurs at tremendously fast, chain-reaction speeds, and in a matter of seconds or minutes, an infected life form is converted into one of a few variety of Flood Forms. In the process of reshaping and repurposing a life form, the Flood infection swiftly utilizes the host's biomass as fuel for further sustenance and the generation of new mechanisms, such as the tentacles and musculature of a Combat Form, or the conversion of a host into a Carrier Form and the growth of its payload of Infection Forms.

The Flood is able to infect hosts in other ways as well. Inhaled spores, wounds inflicted by Combat Forms, or Flood biomatter introduced into the body by any other means also appear to begin the process. However, while an Infection Form can transform a host into a Combat Form almost instantaneously, these other methods of infection seem to mutate the host far more slowly, taking at least several minutes for any obvious signs of infection to appear.

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

The Flood can infest any organism, but 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. Large scale assimilation of non-intelligent species seems to occur in environments lacking intelligent life, such as on the Infinite Succor which carried mostly Covenant livestock. In such cases, the Flood will increase its biomass, but remain in a relatively animalistic state.

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 tendrils that pierce the skin of the host and find their way to the spinal cord. They then synchronize with the host's nervous system and gain control of their body, replacing its consciousness with the ravenous and voracious psyche of the Flood. Any useful information present within the host's brain is retained, however, such as battle strategies and technical knowledge. Despite this lingering of certain memories, no trace of the original mind's personality remains; only a simplistic and primal urge to assimilate other species drives the organism after infection.

In isolated cases, such as when the Flood seek very specific information from an individual's mind, they utilize an alternate, undescribed method of infestation that does not kill the host immediately; this allows the Flood to "burrow" into the host's mind, eventually giving them access to the entire scope of the host's memories. This has only been specifically seen when the Flood sought information from Captain Jacob Keyes, such as the location of Earth or a starship that would free them from Installation 04. In addition, if the Infection Form that initiates assimilation is extremely aged or damaged somehow, the host may remain alive and conscious; Private Wallace Jenkins suffered such a fate, remaining alive and fully aware of his situation despite the mutation of his body, even exhibiting limited control over his body at times when the Flood instincts were dormant.

If the Flood have amassed enough sentient hosts to form a Gravemind, then their behavior becomes far more coordinated and the Flood as a whole becomes much deadlier. The Gravemind may also speak directly through combat forms and pure forms under its control.

Physiological
While baseline neurological assimilation is the same regardless of host species, Flood infection 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.

The Flood sense their surroundings using specialized tentacles, which are tipped with red ganglia and usually protrude from the chest area of the Flood form (or head, in the case of Pure Forms). Exactly how these organs work is unknown; they may provide visual or chemical information, or perhaps a combination of both.

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 the disease "Boren's Syndrome", a neurological condition where radiological exposure causes electrical anomalies in the host nervous system, similar to some of the effects caused by several biologically enhancing drugs used in the ORION Project, the precursor to the SPARTAN Program. In the case of both neural frequencies of the host are scrambled, apparently 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 and consequently infect them; therefore unable to proceed to a large-scale physiological mutation, although Flood DNA is sliced into host's DNA, apparently imparting the host with increased regenerative capabilities, although this has not been independently verified. Avery J. Johnson, having Boren's Syndrome, escaped the containment facility where The Flood attacked two squads of UNSC Marines. However the Boren's Syndrome was a story invented to cover his past in the ORION Project and the side-effects of his (experimental) enhancements, so the validity of each as a countermeasure against Flood infection is questionable. 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 most effective way to stop a large-scale infestation 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 last resort was the use of the Halo installations, which destroyed all sentient life in the galaxy and starved the Flood into extinction. 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. Unfortunately, at the same time, specimens of Flood spore cells were kept for analysis on the Halo Installations, allowing the threat of the Flood to be merely forestalled, not eradicated.

During the Human-Forerunner War, humanity (who at the time was a technologically advanced, spacefaring race) discovered another way to defeat the Flood. Using genetic engineering, a third of the human population was altered to carry a set of specially designed genes. This group of humans was then allowed to become infected by the Flood. Once the genes were exposed to Flood DNA, they triggered a cascade of cell destruction that spread throughout all Flood biomatter. The majority of the Flood died off as a result, although they would return in full force ten thousand years later.

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, which were specially designed to counter the Flood, indicating that the Forerunners had determined energy weapons to be the best 'basic countermeasure' available to fight the Flood. It may be a side-effect of the Flood's conversion of the body into biomass, breaking down cellular integrity and increasing flammability.

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 are now controlled by 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.

Flood forms
The Flood is composed of a variety of specialized forms, each filling a different role in combat. The most basic Flood form is the infection form, which infects hosts by hijacking their bodies and nervous systems. Infected hosts then undergo a rapid series of transformations, becoming lethal combat forms. Combat forms are used to subdue and secure additional hosts for infection, and once they have outlived their usefulness as warriors, they transform into carrier forms, spreading the Flood by spreading more infection forms.

Once the Flood has amassed enough bodies to create a Gravemind, it is capable of generating "Pure" forms, which are created solely from assimilated nutrients and biomass. Pure forms can spontaneously mutate between three configurations: Stalker, Tank and Ranged forms.

Combat behavior
While possessing an unusual sentient intelligence, with Combat Forms still retaining extremely high-level cognitive functions such as vehicular operation, traits such as dexterity and complex 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's strategy is simple: They throw themselves at potential hosts in huge numbers and with any and all weapons available. Flood tactics are based primarily on asymmetrical warfare: the Flood uses all types of its different forms to kill the enemies that it sees. Infection forms swarm across the battlefield in waves, reanimating any corpses they can find, while Combat forms attack and kill any hosts in sight. Pure forms, if present, provide support to Combat forms in a number of ways: Stalker forms move to strategic positions and mutate into either Tank or Ranged forms, which can destroy enemy vehicles or fortifications or provide suppressing fire from afar.

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 Minister of Etiology, 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 has made the assimilation process extremely brief: hosts killed in a short firefight can be reanimated within seconds to minutes by an infection form, and live creatures can be mutated into fully developed combat forms within the same time frame. Sometimes the rate of the mutations can vary in very rare cases: 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 at a restrained and constant rate. Though never seen within the story, it can be assumed infection forms were involved in infecting a victim. Before being killed to prevent him becoming a Flood Combat Form, one of the Sangheili accompanying Rtas 'Vadumee has a wound in his chest that is consistent with the entry point for an infection form.

The Prophet of Truth in his final moments was infected by the Flood and began to mutate, but the rate and nature of his transformation was not consistent with the usual process when a sentient being is taken by a Flood Infection Form. It is likely that this was influenced by the nearby Gravemind, who was either controlling the Infection Form's activity, or controlling the mutation of the Flood spores within Truth's body.

Notable victims

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


 * Prophet of Mercy
 * Prophet of Regret
 * Prophet of Truth
 * Bok

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 one or two 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 can then infect whole Combat Forms if their body still resides on the floor. You can also head shot some combat forms with the Battle Rifle or a Magnum.

Plasma weapons are also very good to have when fighting the Flood, since the plasma melts the bodies. The Sentinel Beam is also very accurate when shooting the Flood because it also melts their bodies which kills them faster.

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 they 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."

Changes from Halo: Combat Evolved to Halo 2

 * The Flood can make screeches and roars, which were not present in the first game. Although they still continue to make low growls upon sighting John-117.
 * Skin is more textured, giving the Flood a more realistic and disgusting look. The faces of the human forms are also more readily recognizable.
 * 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. This could be because of a Gravemind or merely the AI code got updated.
 * 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 the players vehicle, and then will proceed to melee the player.
 * Infection Forms appear less textured and detailed than in Halo CE.

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 ninth and tenth missions, Cortana and Halo.
 * 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: CE and Halo 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.
 * 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, though that could have just been over-compensation for the overhead view of the game.
 * 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.
 * Carrier Forms don't always explode upon death.

Trivia

 * 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 its 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 were 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 armor, 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 billions before him.
 * The name "the Flood" is a reference to the biblical story about the Great Flood in which god destroyed all life except for that which was carried by Noah's Ark.
 * In Halo 2, Combat Forms will go berserk if you kill their allies near them. They drop their weapon and charge at the nearest enemy, mirroring the actions of a beserking Brute
 * In Origins, during the scenes in which the Flood attacks the Forerunner planet, there appear to be many types of Flood, from very large, near-Gravemind-like ones that could level buildings, to flying versions of Infection Forms. In addition, some of the variations seen in Origins are based on concept art for the original game. One explanation for this may be that the Flood had acquired many species. Another explanation may be that the Flood had grown so much, and the Gravemind was so powerful, that it could create Pure Forms for whatever purposes he needed.
 * Sergeant Avery J. Johnson, and John-117 are the only known humans to survive a direct assault by an Infection Form without aid. This is due to the fact he is a member of ORION and their genetic makeup is different than that of a regular human being.
 * The Flood seems to be the reason for the "Gore" label on the ESRB rating. All three games of the Halo trilogy are rated for "Blood and Gore, Violence"; But Halo 3: ODST, which uses the exact same engine as Halo 3, is only labeled for "Blood, Language, Violence", as the Flood are one of the only absent enemies. Halo: Reach was also rated for "Blood, Violence", as the flood were absent.
 * The Flood was named #45 on IGN's Top 100 Videogame Villains list.

List of appearances

 * Halo: Combat Evolved
 * Halo 2
 * Halo 3
 * Halo Wars
 * Halo: The Flood
 * Halo: First Strike
 * Halo: Ghosts of Onyx
 * Halo: Contact Harvest
 * Halo: Evolutions - Essential Tales of the Halo Universe
 * The Mona Lisa
 * Human Weakness
 * The Forerunner Saga
 * Halo: Cryptum
 * Halo Legends
 * Origins
 * Iris