Flood

"One single Flood spore can destroy a species."

- Rtas Vadum

The Flood, (Latin Inferi redivivus meaning "The dead reincarnated"), and The Parasite by the Covenant, is a species of parasitic organisms that thrive by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood presents the most variable faction in the game, as it can transform Humans and most Covenant species, such as Elites and Brutes, into Combat Forms. Infected Humans can be transformed into Carrier Forms as well. The Flood also devoured most of the Forerunners during the approximately 931 year long Forerunner-Flood War.

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 in 98,379 B.C.E. when they encountered the Forerunners on the planet G617g, although, it's still unknown where the Flood actually originated. 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 hollow 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.

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 infest 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.

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", by transporting Humans and various other races to the safety of the Ark. The Forerunners, for scientific study, kept some Flood specimens in high-security laboratories. At this time, a Forerunner called the Librarian was busy cataloging and storing every form of sentient life aboard the Ark.

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.E, 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.

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.

Harvest
Traces of The Flood were discovered by the Unnamed Arbiter (Halo Wars) at the beginning of the Human-Covenant War under the surface of Harvest in an unknown Forerunner sturcture. Although traces were found, it is unknown if there were actually living Flood in the structure. Although there is a Flood presence in a trailer for Halo Wars.

Release
John-117 fends off the oncoming Flood Combat Forms and Infection Forms on Installation 04. 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 scrutiny 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. The AI construct aboard the Human vessel 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 scatter 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 brutal truth through Private Jenkins' recording: this was no Covenant weapons cache. He then proceeded to escape, escorted 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 releasing 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.

Once again the menace of the Flood had risen...

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' conversion 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 loses and rapidly losing ground. Meanwhile, the Flood was 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 Master Chief 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 Halcyon-class Cruiser, 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 from the Flood. The fact that a single Flood form could can cause such destruction has been later supported in Halo 3 when Rtas' Vadum 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, 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 infested; 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, were dispatched by the High Prophets to "silence" Sesa 'Refumee, the leader of the Heretics stationed within the mine who was encouraging dissent against the Prophets. While the force attempted to track down Sesa 'Refumee, assasinate him, and destroy his forces, by accident or design, unleashed The Flood that were held within the station, and there was absolutely nothing they could do about it.

During the Arbiter's pursuit of Sesa 'Refumee, during which the Arbiter neutralized several weapons emplacements, and enemy Banshees, the Arbiter, 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 The Arbiter 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". The Arbiter's allies were slowly picked off as Flood attacked the elevator, with Sentinels continually hovering overhead and firing lasers into the chaotic fray. Finally, the Arbiter left the elevator into a second wide laboratory hall, where the Heretics attempted to lock out both the Arbiter and the Flood. The Heretics' plan was not flawless, however, and handful of Heretic Elites were still present in the second laboratory when the Flood entered, these Elites were presumably infected while a horrid tide of Infection Forms, Combat Forms, and Carrier Forms lurched into the hall, and the Arbiter was forced to defend himself against them until the tide stilled and the doors were opened.

The Flood would not be contained in rooms, and attacked both the Covenant assassination force and the Heretics themselves. Their voraciousness 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. The Arbiter 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, the Arbiter 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 the Arbiter 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 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 involved more in hiding than 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 a several Enforcers.

However, two external forces, the Covenant and the United Nations Space Command, 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 Arbiter, 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 Brutes, Grunts, and Jackals 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. The Arbiter 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 the Arbiter in a Phantom drop ship, where the Arbiter 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 number of Marines to fight through the Flood-Infested Wall, but they were grossly overrun. The Arbiter 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 Elites in the lower levels of the Flood-infested Wall, although those were killed as well. Finally, the Arbiter 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 the Arbiter, 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 even 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 The Arbiter 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 the Arbiter in forging towards the center of the Flood-infested Quarantine Zone, so that the Arbiter 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 the Arbiter, 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 the Arbiter 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 the Arbiter, 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, the Arbiter prompted the Gravemind to show him, by "stopping the key from turning.", transporting SPARTAN-117 to High Charity, and the Arbiter to Delta Halo's Control Room respectively.

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. 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 Cortana ''

''"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 Brute fleet escaping to the Ark, a Flood infested Covenant cruiser appeared out of Slipspace and crashed into the city of Voi. 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 Arbiter fought their way to the ship, the Shadow of Intent arrived and deployed an Elite task force to aid UNSC marines. Master Chief, with the help of the Elites, 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, Master Chief boarded a Phantom while the Elites went on to glass the infected areas. The forces, fulfilling their mission of removing the Flood from Earth, retreated back to the Elite 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 the Arbiter's and Rtas 'Vadum's Elites through the Portal.

On the Ark, while attempting to lower the shield surrounding the Ark's citadel, the infested High Charity made a slip space 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. The Master Chief 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 Gravemind destroyed, and all the Flood on the Ark gone, it is more 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, seeing as 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 got here sparks up even more mysteries. Perhaps 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. For now, the Flood are seemingly defeated. Also, after the activation of the replacement Installation 04 in Halo 3, Gravemind sent one final telepathic message to the Master Chief, claiming that this was "another long stretch of time", suggesting that it may still exist, despite the destruction of High Charity and the Ark. It is also a possibility that it is in fact referring to other Graveminds, because as stated in several Terminals, there was more than one present during the Flood-Forerunner War.

Developmental Stages
The Flood go through 4 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 2.)
 * 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 stage 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. (Not yet seen).

Biology


The Flood, a parasitic species, infects individual organisms and causes them to undergo different stages of organized transformation into various "forms". A Flood's chosen form depends on the species of creature infested, and the usefulness of the infected body. All Flood biomass is made up of the differentiable Flood Super Cell. The Flood also exhibits a strange ability to alter the environment to suit their own needs.

The only known pathway for Flood reproduction and survival is by the infestation and assimilation of other species.

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; therefore the more cunning the enemy, the more dangerous the Flood will be.

Neurological
Infection forms selectively target other species that possess sentient intelligence and are of sufficient biomass, and can infest living or 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 Brutes, Humans, and Elites 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), and "No skill" (IWHBYD skull required). 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, Brutes, or Elites, 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 hit box-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. 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. 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
Because of the millennium of Flood existence, it is believed that the Flood have evolved so extensively that there are few direct countermeasures against infection.

There is no confirmed way to effectively stop a Flood infection when an Infection Form attacks a host. However, a possible exception is to have 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 negative manifestations to the host. However, it has been suggested that during Flood infection, the infection form is unable to rewrite the psychology of the host, and is unable to proceed to large-scale physiological mutation as well for an unknown reason, although Flood DNA is spliced into host DNA, even imparting the host with increased regenerative capabilities. Avery J. Johnson, having Boren's Syndrome, escaped the containment facility in which The Flood attacked two squads of UNSC Marines. However, 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.

While Boren's Syndrome left untreated may leave you immune to the Flood, leaving it untreated usually equates to death within a couple 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 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.

The only other way to stop an infection is to kill the infection form itself before it digs into it's host, as done commonly in-game.

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 Flood forms who ever 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, and infecting potential hosts if too many are ingested or inhaled.

Infection Form


The Infection Form is the second most basic and simple Flood form, and is larger only than the Flood Spores, albeit 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 creatures attack in dozens or even hundreds at a time. Their discovery on Halo indicated that either they can remain dormant for countless millennium until a suitable host awakens them, or they were kept alive for research purposes. It is a small, spherical organism, with a small, sagging, brown horn-shaped projection on its back, and tentacles projecting from its spherical surface.

These tentacles propel the Infection Form through sinusoidal motion, and are used to bring the Infection Form towards its target. Upon reaching its target, it will attempt to infect it by extending its tentacles, penetrating its skin, and contacting the host's spinal cord, bringing about a successive series of neurological and physiological changes to the host. Once the host’s neural signals and bodily functions have been assimilated, the infection form deflates in order to fit between the host’s heart and lungs when it digs itself into the chest of the host’s body. . Once inside the host’s body, the infection form spreads its tentacles throughout the host’s body and assumes total control. It then morphs the host into a combat form. If the combat form is destroyed, the infection form may or may not escape from the cavity. The changes made depend on the species assimilated by the infection form. Infection Forms are also known to age, as can be seen by the Elder Infection Form.

However, energy shielding provides some protection against an Infection Form, popping the creature in a small, but damaging, blast of noxious gas. The only known human survivors of a Flood infection attack were Staff Sergeant Johnson, supposedly because of Boren's Syndrome, and Master Chief. Johnson's potential status as a Spartan-I means that his survival may have been due to his augmentations, rather than the Boren's Syndrome cover story. Infection forms are extremely dangerous to unshielded enemies even in small numbers: a single contact between an infection form and an organism lacking a personal energy shield will lead to a gruesome assimilation within minutes. But in Halo 3, the coordinated-stage infection forms transform their victims into Combat Forms very rapidly in just a few seconds. Infection forms can't infect beings with strong armor, such as Master Chief. The addition of a personal energy shield is a tremendous advantage in fighting Infection Forms, making them dangerous only in significant numbers: the infection forms must first explode when reaching the potential host, lowering the shields with repeated detonations before seizing upon the unshielded host's armor, and then proceeding to assimilate, requiring a tremendous number of Infection Forms to breach the shielding and combat armor. The Infection form is most easily defeated with the M6D pistol, as the explosive rounds can take out multiple Flood at once, in Halo 3 dual wielding Brute Maulers can effectively take out a pack of infection forms. Or one could use a plasma grenade to stick them all at once because, if you pop one that's inside or near a pack, it will cause a chain reaction.

Combat Form


After an Infection Form has successfully developed into 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 only mutating Humans, Elites and Brutes, not bothering to utilize Grunts, Drones or Jackals presumably because of their physical weakness and lack of significant biomass and combat capability, instead turning lower creatures into Carrier Forms and, possibly, because of the fact that it would take a longer amount of time to create a Carrier form, and grow infection forms inside.

Combat forms are more dangerous at close range, utilizing their whip-like mutated arms to lash out with scourging blows of significant force, even stronger than a SPARTAN-II warrior augmented with strength-amplifying amorphous circuitry. A single blow can lower a MJOLNIR Armor's personal shields by half, and a few melee swipes are needed to drop a lowly Grunt or Jackal, while sufficient numbers can down even an Elite, Brute, Marine, or Spartan.

combat forms can also use any weapons or vehicles that their host body might once have used, although they are poorly proficient with almost all firearms and are clumsy with vehicular operation, as seen in their reckless driving in Halo 2. They have been known to utilize active camouflage as an Elite form. Of most infected Combat forms, Human Combat forms tend to use the weapon the host was using before they were infected, moreso than other Combat forms (Brute or Elite combat forms). Combat forms are rakishly aggressive and lack even rudimentary battlefield tactics, although their sheer numbers can force even powerful enemies to succumb. Infected Elites, Brutes or Humans can be transformed into combat forms within seconds, and corpses of previously infected hosts can be alarmingly reanimated in seconds by other Infection forms, making corpse destruction a useful tactic when combating the Flood, and is even recognized by the Forerunner Sentinels, who sometimes incinerate infected corpses to ensure that they can not be reanimated. If you destroy the body, then the Flood cannot reuse the body for infection. Players in Halo 3 often use weapons such as the Gravity Hammer to quickly eliminate Combat Forms, seeing as the Hammer destroys their bodies as well. Since the infection form needs sizable amounts of Calcium to survive, infection forms do not assimilate Grunts, Jackals, and Drones, which probably lack the appropriate Calcium.

Carrier Form


The exact origin of Carrier Forms is unknown, although they are theorized to be combat forms that are too injured, damaged, or aged to be useful for future combat, or smaller and weaker life-forms (like Grunts and Jackals) who have been infected but lack the calcium mass to serve as combat forms, or both. The purpose of a Carrier Form is to nurture and disseminate Infection forms upon attack or proximity to a host.

Physically, they are a large, bloated sac situated upon a torso and a pair of waddling legs that slowly transport Infection Forms. Their bloated "sac"-like form is a result of the many Infection forms crawling around inside them. In combat situations, once they are within close proximity of a potential host, or when their sacs are perforated even with a single projectile, the carrier form is forced to its pudgy knees while the sac exponentially swells with gas for several seconds, then spontaneously detonates into fleshy scraps, hurling infection forms into the air. The detonation of itself is extremely damaging, and its gas-propelled dissemination of infection forms facilitates easier assimilation, as it hurls infection forms in all directions, and stuns the potential hosts with the concussive blast. This explosion also has the potential to set off any stray grenades, making it even more deadly. Originally, a shielded form of the carrier had been designed for Halo 2.

In comparison to Halo: Combat Evolved and Halo 2, Halo 3's Carrier Form is much more difficult to destroy, and releases more Infection Forms; however, if set on fire, or stuck by a plasma or spike grenade, no infection forms are released, making flame and plasma-based weapons excellent for dispatching such forms. The explosion caused by a Needler is just as effective. But ballistic rounds will burst the sack and release Infection forms.

It is sometimes prudent to pop a Carrier Form while it waddles toward a target, as nearby Combat Forms can be damaged by the explosion.

Pure Form
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. 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 was 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.

Pure Forms come in 3 different types, and can transform between these forms within seconds.

Stalker
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. 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.

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. It is especially weak against plasma, spike, and firebomb grenades. 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, on Halo 3, the Ranged Form will drop Spike Grenades(on Legendary).

Tank Forms
Tank Forms are large and easily spotted. Unlike the other Flood Pure Forms, they can not 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 oversized 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.

Juggernaut Form


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. It is named for its size and its power as the Flood Juggernaut is very large, about twice as tall as the Master Chief and Elites, 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 one can imagine with its long legs and long tentacles, that it would have little problem snatching an Enforcer out of the sky and tearing it apart. 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 was modded into the level High Charity. They were not in the final version of Halo 2, and are only accessible on High Charity through modding. It is wondered why it was cut from the game, and what it came from. Most speculate that it was an early version of the Flood Tank Form. The weapon of choice to use against them is the M-19 SSR MAV/AW, believed to be the reason the rocket launcher is on the level High Charity. They were never meant to be incorporated into the game, but some people found the glitches in the game that allowed them to fight the Flood Juggernaut.

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 less 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 all 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 defeated fleets of thousands! Consumed a galaxy of flesh and mind and bone!"

-The Gravemind on the level Halo

The Gravemind is the leading 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, since the statement above supports this. 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 afterwards, 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.

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 follows a zealous, 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 with the infection, combat, carrier, and tank forms to rush the enemy and quickly eliminate them. The rangers try to support their zealous comrades with a ranged attack. The stalkers attempt to form into a tank or ranger. 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 ordinance.

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 a shielded 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 firearms 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, with 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 Elite was assimilated, all the Flood could open any doors and access any system on the ship immediately afterwards, 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, "[t]he 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.

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 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 Elites 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.

Halo: Combat Evolved
It is best to use the MA5B Assault Rifle, Shotgun, or M6D Pistol against the Flood. The pistol is generally not recommended for mass engagement, as it's 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. 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.

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. 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.

Halo 3
NOTE: The Incendiary Grenade instantly destroys all Flood forms and is the one true weakness of all 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.

Infection Forms
Infection Form

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. 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 Brute 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 countpart, the Covenant Carbine. Using sticking grenades such as Plasma Grenades and Spike Grenades also dispose of these enemies quite quickly. In close-combat, the melee of most enemies or the famous Energy Sword, Gravity Hammer, or Shotgun will kill them in one or two hits. 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.

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 countersniping 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 countersnipers is that it will occasionally guard its vulnerable mouth when shot upon, making it resistant to all forms of weapons.

Halo Wars
Their tactics on Halo Wars are largely unknown, so it is unknown how they will deal with vehicles, though they can obviously infect normal troops. In the one in-game release of the Flood in action, they possess large tentacle like flood forms and large pods that can pin in and attack vehicles, in the case of the Elephant they capture, which may be an in-game objective to rescue.

Halo 2 (from Halo: Combat Evolved)

 * 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. Faces are also more recognizable on humans, a touch many players find scary.
 * 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.
 * Rather than corpses re-animating themselves, an Infection Form will enter the body, then re-animate it.
 * Flood forms can now be beaten or shot into pieces.
 * 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 deleted and kept in 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.

Halo 3 (from Halo 2)

 * Brutes may now be infected and transformed into Combat Forms.
 * Combat Forms are significantly easier to kill.
 * 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 meleeing or shooting. A realistic touch, considering the Flood's weak, rotten flesh.
 * Infection Forms can mutate humans, unshielded Elites and Brutes into ready for action Combat Forms in around seven seconds (a 7 reference), unlike in the first two where it could take days.
 * Pure forms are introduced, consisting of the Tank Form, the Stalker Form, and the Ranged Form.
 * Flood Growth Pods are introduced, growing on walls in ships and buildings.
 * Flood Dispersal Pods are introduced.
 * Gravemind can 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.
 * Combat Elites are stronger because of the shield.
 * Carrier Forms are significantly more difficult to kill with projectile weapons, but they are easily destroyed by Plasma Weapons (3 bolts from the Plasma Rifle on Normal would do the job).
 * The Shotgun is significantly weaker against Flood.
 * 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.
 * Flood Infection Forms have a different appearance in Halo 3.
 * 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.
 * If you hide from the Flood in crouching position, Gravemind will speak through his minions and say, "come to me".
 * The infection of a Combat Form can happen when the creature is alive or dead.
 * Running over Infection Forms in a vehicle can result in a bumpy ride and the Flood will not pop.

Known Victims

 * Prophet of Truth
 * Captain Jacob Keyes
 * Wallace A. Jenkins
 * Mendoza
 * Kusovai
 * Private Riley
 * Minister of Etiology
 * Prophet of Regret
 * Prophet of Mercy

Trivia

 * Flood have been confirmed to be in Halo Wars; however, they will be a non-usable faction.
 * The Flood are first seen in Halo: Combat Evolved through Private First Class Jenkins' helmet cam or through the eyes of the player although they are un-attackable and appear as yellow dots on the player's radar.
 * When the I would Have Been Your Daddy skull is activated in Halo 3, two things will happen:
 * In the background, murmurs can be heard from Combat Forms that sound like "Braaiinnssss..." This is a reference to the fact that Flood are often considered "Zombies"
 * In Halo 3 their noises sometimes sound like: "I see you" if you are hiding in a corner.
 * The first known Human to be infected by the Flood was a UNSC Marine named Private Riley, this will undoubtly change once Halo Wars is released.
 * The Flood Juggernaut was a Flood Form intended to be featured in Halo 2, and was found in the game's code, but was never featured in the full game, much like the Engineers in Halo: Combat Evolved. They had respawn points in the beginning of the level "High Charity" in Halo 2, and can only be fought through modding.
 * In the level Cortana, a vein-like structure can be seen on a wall next to the second platform before you jump in; this vein could carry calcium to help cultivate and feed the Flood as they grow.
 * Blowing off a Flood Combat Form's limbs in Halo: Combat Evolved would cause the form to trail the player, as it would want to attack, but would not possess the appendages to do so, a trick often referred to as "Flood Friends", "Harmless when Armless", or a "Flood Buddy", although occasionally, the Flood form would attack with invisible arms. In Halo 3, using a single-shot weapon, like the BK for example, if you aim at the shoulder on any non-elite infected forms, they will lose the arm and constantly push you. If they are far enough away from you however, and they have no arms, they can still hit you by lunging. If you leave armless flood alone for enough time they will explode with no attack upon it at all.
 * 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'.
 * Combat Forms and Pure Forms seem to have the ability to "talk" in Halo 3, although they only use this ability to serve as mouthpieces for the Gravemind.
 * In Halo 2 and Halo 3, Combat Forms may suffer a head shot if hit in the chest, quickly destroying the Infection Form in the chest cavity. This makes it easy to take them down using Battle Rifles or Pistols, but not overly useful unless there aren't any Infection Forms around to reanimate those downed in that way.
 * 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, its 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 (evil) video game villains of all time."
 * In Halo 3, you can stick a corpse just before it is re-animated. Once animated the sticky grenade seems to have disappeared (This works with all sticky grenades including the Incendiary Grenade). (This also happens when you stick a "pure" form Flood in mid-transformation)
 * In Halo 3, occasionally, you might come across Spec Ops Elite Flood Combat Forms that can take a lot of damage, such as being able to withstand three Energy Sword lunges, and even a couple Gravity Hammer hits. In some games, around every one out of five Flood-infected elites will have this.
 * In Halo 3, Flood are almost the same in every difficulty; the only difference being their strength. They can still be taken down by one melee on even Legendary.
 * In both Halo 2 and Halo 3, once a Combat Form's corpse is incapacitated (i.e, the Infection form inside the cavity is shot) the lifeless body will give out a scream or growl.
 * One early theory of the Flood's origin was that they were a Forerunner experiment that went horribly wrong, but this idea has been proved wrong. It is more likely that the Flood had an extra-galactic origin.
 * Private Mendoza can be found in the Halo: Combat Evolved level 343 Guilty Spark, despite being attacked by the Flood. He exists as an Easter egg in the level.
 * In Halo 2 it takes several melee hits to bring down a Flood unlike one or two hits like in Halo 3.
 * Also, The Flood in Halo 2 take several melee hits to "burst apart" after you have killed them, unlike Halo 3 where it takes only one hit to do so.
 * They were named the 7th most bad ass zombie characters in all video games by E3.
 * If you shoot off both arms of a combat form, it will wander around and then fall apart. After it falls apart, the infection form inside it will come out.
 * Throughout the Halo series, it has become increasingly easier to kill Flood with melee attacks, in Halo:CE it was impossible to kill a Combat Form with a melee strike, in Halo 3 it takes only a few hits, depending on difficulty.
 * Infection and Tank Forms can be seen on the multiplayer map Cold Storage, however they will not harm you.
 * In the multiplayer level Isolation there are Flood Growth Pods, there are no Infection Forms in these Pods, they still however cause damage if a player is too close when they burst.
 * In any flood level if you have the IWHBYD skull activated it will say brains or if you are hiding in a corner they will say I can see you