Flood

The Flood, (Latin Inferi redivivus, meaning "The dead reincarnated" ), and also known as The Parasite by the Covenant, is a species of parasitic organisms that thrive by consuming all sentient lifeforms of sufficient biomass and cognitive capability. The Flood presents the most variable faction in the game. This includes Humans and most Covenant species, such as Elites and Brutes. The Flood also devoured most of the Forerunners during the Forerunner-Flood War.

Forerunner-Flood War
The Flood are believed to have arrived to the galaxy more than 150,000 years ago prior to 2552 in 98,379 B.C., when they encountered the Forerunners on the planet G617g, although, it's unknown where the Flood actually came from, planet-wise. 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, soon moving to similarly infest 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 ground. While at first the Flood demonstrated a lack of even "basic cohesion", they had "numerical superiority"; billions of forms to dedicate for the assault and every member of the Forerunner population a potential new kind of Flood form. 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 initially with Pyrrhic 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 afterwards, 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 spawn a Gravemind form, a creature that embodied a collective sentient intelligence for the Flood species, and one that could coordinate the Flood swarms in active 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. Some Flood specimens were kept by the Forerunners for scientific study. 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 installations, known as Fortress Worlds, or Halos across the galaxy. Because the Flood were parasitic, and their survival was immediately 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, destroying almost all sentient life forms. However, the human race and many other species throughout the galaxy on the Ark survived from the firing of the Halos because it was out of range of all 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.

Release
Although large numbers of the Flood were starved by the first firing of the Halo Array, 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) to evade Covenant pursuers. This led both species to Installation 04, and in turn, the Flood on Installation 04. After the crash of 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, their pursuers, Covenant cruisers, among them the cruiser Truth and Reconciliation, amassed their ground forces on the surface and soon discovered an underground Forerunner 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, that of a Flood storage facility. Though the some of the Flood managed to escape, the Covenant, terrified, eventually pushed the Flood back in. It is unknown whether a few still escaped. the flood escaped due to a leak in the library thus 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 a group of Marines, Fire Team Charlie to seize it. When the team arrived at the swamp however, they found the facility is 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, and 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 with AR fire, he believed them to be allied forces. However, after he opened the door that Captain Keyes had foolishly unlocked, he learned the brutal truth through Jenkins' video: 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 Teleports John 117 into the Library and quotes "The Flood is spreading. We must hurry!"

The few surviving Covenant soldiers left were scattered and disorganized, and put up little resistance to numerous mangled Covenant corpses, disfigured by an unknown and mysterious force. It is presumed that the the Covenant occupants of the research facility had moved through the structure in hopes of finding Forerunner artifacts, 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, and proceeded 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, which 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 in the form of Ghost attack vehicles, Banshee fighters, and Wraith heavy tanks, the weaker Grunts and Jackals succumbed easily to the Flood, and the few Elites who remained were outnumbered and as a result were 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, with heavy casualties. However, one of the attacking combat forms, a UNSC Marine, Wallace A. Jenkins, one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by an 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 able to betray the Flood through inarticulate 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, thus preventing 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 the UNSC were sustaining heavy loses and rapidly losing ground. Meanwhile, whole areas formerly held by the Covenant were being occupied by the Flood, 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 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 Brain Form aboard the CCS-class Battlecruiser, Truth and Reconciliation, a Covenant warship that the Flood were relentlessly attacking. While 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 through Keyes' skull to get the Captain's CNI.

The Covenant Flagship Truth and Reconciliation was a major point of concentration between the Covenant Special Operations units (SpecOps Elites and Grunts) 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 liftoff 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 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 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 a species."

Defeat
The Flood appeared to be interested in seizing space-capable vessels, in the hope of spreading itself away from the Halo installations and into the stars: they attacked the UNSC Pillar of Autumn, also. 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, infested, and 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 come into 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 from detonating the wrecked warship's reactors and escaping in a C709 Longsword-class Interceptor while the thermonuclear detonation severed the ring world, destroying it and its capability to annihilate all sentient life and killing 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 Elite Special Operations Elites and Special Operations 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 that was encouraging dissent against the Prophets. While the Covenant Special Operations force attempted to track down and assassinate him, Sesa 'Refumee and 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, in which the Arbiter neutralized several weapons emplacements, and enemy Banshees, the Arbiter, Rtas 'Vadumee, two Special Operations Elites and two Special Operations Grunts entered into a laboratory, and the Heretics deliberately locked the doors: most likely by design, in order to crush the Covenant 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 stochastic 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: a handful of Heretic Elites were still present in the second laboratory when the Flood entered, and those Elites were annihilated while a horrid tide of Infection Forms, Combat Forms, and Carrier Forms lurched into the hall as 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 was continued to pursue Sesa 'Refumee, but was impeded by large waves of Flood crushing outnumbered the Heretic Elites, Heretic Grunts, and 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. 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 outbreak and that any remaining Flood (and, presumably, remaining Heretics) would be killed once the facility entered the planet's Nitrogen core.

Battle of Installation 05
Unlike on Installation 04, where the Flood had been contained by the Sentinels 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 created large amounts 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 biological "airlock" that would prevent the Flood from spreading any further. This multi-tiered defense was presided over by hundreds of Sentinels and Sentinel Majors, and a handful of 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 an Elite Strike Team of Special Operations Elites, commanded by Special Operations Commander Rtas 'Vadumee and led into battle by the Arbiter, to retrieve the Index in a multi-staged assault. However, first, the Covenant 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 defending positions instead of attacking ones. 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 Containment Shield was activated by the Sentinels, 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. Tartarus deployed the Arbiter in a Phantom drop ship, where the Arbiter demounted, 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 residence in the Flood-Infested 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 Flood-Infested 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 arced chasms that periodically punctured 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 may reach the Library and take the Sacred Icon.

The Flood, despite a significant vehicular and infantry presence in 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 through the battling Flood and Sentinels led by the Arbiter. Later In Amber Clad is 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 through the use of the captured Monitor, 2401 Penitent Tangent, and the semi-revived Prophet of Regret. 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 were 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 words of comfort the Prophet of Truth delivered to the city's population were regularly rebutted by Gravemind 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 Truths fleet to Earth as well as transporting the Prophet of Truth. Cortana, who had to stay behind to manage the situation should Installation 05 be fired, was left in the clutches of Gravemind, who could potentially learn all possible Human knowledge from her.

Earth, The Ark, And The Flood's Defeat
''"Child of my enemy, why have you come? I offer no forgiveness; a father's sins pass to his son."

- Gravemind on the level Cortana  "I am a timeless chorus. Join your voice with mine, and sing victory everlasting."

- Gravemind on the level Floodgate ''

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

- Gravemind on the level Cortana '' At the conclusion of the Battle of Voi, with the Prophet of Truth activating the Portal, and the Brute fleet escaping to the Ark, a Flood infested Covenant cruiser slipped 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 Flood forces, including newly encountered Pure Form Flood 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 slipspace jump and smashed into the Ark, releasing the Flood inside the structure and causing an infestation on the newly built replacement for the destroyed Installation 04. After retrieving Cortana from the derelict High Charity, SPARTAN-117 was able to kill all Flood present on the Ark and the replacement Installation 04 by activating the unfinished Halo using the Index Cortana retained from the first Halo. The ring's activation also presumably killed the Gravemind. The Master Chief also destroyed High Charity by blasting 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 to destroy the flood.

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 as that Installation was never destroyed) or any other remaining Forerunner Installations. The fact that the Flood were never originally from this galaxy to begin with further complicates the issue. 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," for it, 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 were 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 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 at the Milky Way had descended from Intergalactic level Flood. (Not seen yet).

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 exhibit a strange ability to alter the environment to suit their own needs.

The sole 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. 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 like "Where is he"(Uncommon), "Don't hide from me"(Common), and "You will be ours"(Occasionally). 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, punching through and exiting 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 multifurcates at its terminus, with strange multiple angular hand-like entities replacing the hand. The musculature of the combat form dramatically changes as well, and the combat 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 a host is attacked by an Infection Form. 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 two squads of UNSC Marines were attacked by The Flood. 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 researched on to produce a cure.

Since the Flood are 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, the smallest Flood type of organism and 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.

Infection Form


The Infection Form is the most basic and simple Flood form, and is the smallest, albeit the easiest to defeat, although it is believed to be the most essential, it is the Flood's method of infecting and assimilating other species as these creatures attack in dozens or even hundreds. Their discovery on Halo indicated that either they can remain dormant for countless millennia 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 projecting from the top of 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. As these changes happen, the infection form will deflate its gas bag, pull itself into the host body, and spread its tentacles throughout the body. However, energy shielding provides some protecting against an Infection Form, popping the creature in a small, but damaging, blast of noxious gas. If you pop one that's in a pack it will cause a chain reaction.

After infecting viable hosts, the infection form resides in the chest cavity. When the combat form is destroyed, the infection form may or may not escape from the cavity.

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 horrible 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 wield Brute Maulers can effectively take out a pack of infection forms. Or use a plasma grenade to stick them all at once.

The only known human survivor of a Flood infection attack was Staff Sergeant Johnson, 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.

The Infection Form penetrates through flesh into the spinal cord. Once the hosts neural signals and bodily functions have been assimilated, the infection form deflates in order to fit between the hosts heart and lungs when it digs itself into the chest of the hosts body. Once inside the hosts body, the infection form spreads its tentacles throughout the hosts body and assumes total control. The infection form then begins a series of neurological and physiological changes to the hosts body, turning it into a combat form. 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.

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 often times 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 make a Carrier form and the growing 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 few melee swipes are needed to drop a lowly Grunt or Jackal, while sufficient numbers can down even an Elite, Brute, Marine, or Spartan.

As mentioned below, combat forms can use any weapon that their host body might once have used, also utilize weaponry or vehicles, although they are poorly proficient with almost all firearms and are clumsy with vehicular operation. They have been seen to utilize active camouflage as an Elite form. Of most infected Combat forms, Human Combat forms tend to use the weapon it was using before it was infected more 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 can not reuse the body for infection. Since the infection form needs sizeable amounts of Calcium to survive, it is seen infection forms skip 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 combative for further use, 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 it releases more Infection Forms. However if set on fire or is hit by a plasma or spike grenade, no infection forms would be released, making flame and plasma excellent weapons against such forms. The explosion caused by a Needler is just as effective. But never shoot any ballistic rounds because it will burst the sack and release Infection forms.

Pure Form
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 was 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 shape of hosts as well as enough complex neurosystems 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.

They can transform into another pure form within seconds; a stalker form usually turns into a tank form when you get close to it, and a ranged form if you're trying to kill it and there's a wall nearby; ranged forms mainly turn back into stalkers to transform into tank forms.

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.

The ranged form is susceptible to the Covenant Needler. It also is VERY useful to use plasma, spike, and firebomb grenades on a ranged form. 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 look like the spikes from a Brute Spiker.

Juggernaut Form


The Flood Juggernaut is the name of a large Flood form (possibly pure) found encoded in the Halo 2 disc. It is named for its size and its power; 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 were modded into the level High Charity. They were not in the final version of Halo 2, and are only accesable 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.

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 retains 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. It is ravenous for the life of all, as most flood such as Gravemind are. Only 1 Infestation process of the Prophet form is seen but yet to be completed (High Prophet of Truth).

Brain Form/Proto-Gravemind


One of the rarest types 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, 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 all of its 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 turned into a Proto Gravemind as he had superior knowledge about the ship and intelligence too. It is destroyed when a squad of Special Operations Elites infiltrated the bridge after John-117 had escaped the cruiser aboard a Covenant Banshee.

Gravemind
"I've beaten 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. 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 have 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 millennia.

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, gravelly 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.

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, such as precise dexterity and overall battlefield tactics, are notably lacking in the Flood, especially in their psychological and combat behavior. While the Gravemind coordinates Flood strategy on a galactic scale, small encounters are not coordinated between individual Flood warriors. The Flood follow a zealous, ravenous strategy, with little target selectivity, aggressively attacking all hostiles in range. Furthermore, Flood tactics are based primarily on asymmetrical warfare: The Flood employ swarms of creatures, often times outnumbering enemies by horrendous factors, compensating for their individual weakness with massive numbers in an attempt to overwhelm enemies and then reanimate their corpses afterwards.

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, as aforementioned, the Flood demonstrate a curious higher intelligence despite their poor combat aptitude and coordination. The Flood collectively learn the intelligence of any species of which even one member is assimilated, and selective hosts with particular knowledge, such as vehicular/starship operation, are still imparted with that intelligence after being infested. 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 CCS-class Battlecruiser 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, many Elites aboard the Infinite Succor were mutated. At times, the mutations can occur randomly and 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 physiological 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 biomasses 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 permutations of the Combat Form as exemplified in the Carrier Form being a further mutated 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, 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 whilst still being potent enough to destroy the combat forms. The Shotgun is the best choice against Flood Forms, as it usually finishes them off in 1 or 2 blows, 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 mass 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 the pistol for far range (killing them before they can see or if they are "sniping") or the Assault Rifle for quick work at point-blank. The pistol is most useful for 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 have proven to be very useful, as well as the Sentinel Beam, Shotgun and Energy Sword. However, Plasma Pistols will be the least useful. Dual-wielding Plasma Rifles and aiming at the arms of the Flood is more than enough to rip them apart easily. These will come off quickly and therefore leaving the combat form literally unarmed. They will waddle about and explode after a few seconds. The SMG is also as good against the Flood as Halo 2, 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 the Combat Forms will kill the Infection Form controlling the Combat Form in a single burst. It is often a good idea to use the Brute Shot melee or Energy sword on "dead" Flood (as well as living Flood) as it will destroy the body and prevent Infection Forms from bringing it back to life. For the best combo use Dual SMGs for Combat Forms and Infection Forms. Keep a Shotgun just in case you are out of ammo on the SMG.

Halo 3
Sticking Carrier Forms with plasma grenades or using a flame-based weapon against them will kill them and their cargo of Infection Forms. Energy Swords are the best weapons to use against Pure Forms as, even on Legendary, they will still kill a pure Tank Form with one hit, but are closely followed by the Brute Shot, as the melee attack kills and incapacitates all forms of Flood in one hit, except for Tank Forms, which take two hits unless you melee them in the mouth. The fact that you can use the melee as many times as you want makes the Brute Shot more useful than the Energy Sword in situations where there are extremely large numbers of Flood. Like in Halo 2, it is also a good practice to melee dead Elite and Brute combat forms in order to prevent Infection forms from bringing them back to life. Gravity Hammers are also good for large groups of Flood, but aren't nearly as effective against pure forms as Energy Swords. The shock wave from the hammer can burst the pods containing Infection Forms, resulting in more enemies. If you manage to get a Flamethrower, they are especially helpful against Pure Forms, as it will almost always kill them instantly, even on Legendary difficulty. Another good way to take care of the Pure Form is a Plasma Grenade in the mouth. The MA5C Assault Rifle is excellent for Combat Forms and Infection Forms alike, though it is unfortunately ineffective against Pure Forms. Wielding two Plasma Rifles is also a good option against the Flood. The weapons accuracy makes them even more greater damage against the Flood. One good weapon for the Flood is to be able to be "reinfected" after being shot a lot. One way to counter that is to get a weapon with a strong melee, (to avoid wasting ammunition), and then melee the body to parts where the Infection Forms can't reanimate them. Some good weapons for the tactic are the Mauler, Spiker, MA5C, Energy Sword, and the Gravity Hammer, just to name a few.

Using a Battle Rifle(Halo 3), if you shoot the chest where the Infection Form is located on a Combat Forms, this will act as a head shot and subsequently kill them instantly. This is also the best way to kill Combat Forms while in campaign scoring, as you will get the headshot medal. A downside to using the Battle Rifle is that it very rarely destroys the corpses, making it possible for them to be reinfected.

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 flood Elites still have their energy shields functional.
 * 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 appear 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) 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 camoflage anymore. Although only Elites in the final level of Halo had this, so the change is hardly noticeable.
 * Flood Combat Forms do not "feign death" anymore.

Halo 3 (from Halo 2)

 * Brutes may now be infected into Combat Forms.
 * Combat Forms are SIGNIFICANTLY easier to kill.
 * 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.
 * 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 could be use to dismember Combat Forms, either by meleeing or shooting. A realistic touch, considering the Flood's weak, rotten flesh.
 * Carrier Forms release significantly more Infection Forms. When they explode, they also don't push the player away with the percussion as much as in Halo 1 and 2.
 * Combat Elites are stronger because of the shield.
 * Carrier Forms are more difficult to kill.
 * The Shotgun is significantly more effective against Flood.
 * If you are hit by an Infection Form while your shield is down it will not only make a crunching noise, but it will most likely kill you.
 * Flood Infection Forms have a different appearance in Halo 3.
 * In the third to last level the Flood helps you on your way to the Prophet of Truth, but then turns against you once the Arbiter kills Truth.
 * Combat Forms can now be destroyed and rendered useless to the flood, by shooting with fire weapons, plasma weapons, or melee.
 * 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.

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

 * On Flood levels such as Cortana, Floodgate, and Halo (Level) are good times to have Catch Skull on, since the Flood do not throw grenades.
 * On the level Floodgate, it is good to put on the Thunderstorm skull on as the only things that will be promoted are Elites which will be on your side.
 * If a Brute Chieftain is infected, his armor will turn blue.
 * 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 in Halo 3, 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.
 * Sometimes, when a Marine is infected, he will grab his head and twist it, breaking his neck; killing him/herself to escape the pain of infection.
 * The first known Human to be infected by the Flood was a UNSC Marine named Private Riley.
 * 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, 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. This was disabled in Halo 2 and Halo 3. This instead results in the death of the Flood prior to the loss of the last appendage, when the infection form comes away from the body.
 * 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 and Pure Forms seem to have the ability to "talk" in Halo 3, although they only use this ability to serve as mouthpieces for 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 Halo: Combat Evolved, it is possible to melee a Combat Form to death, although it will take a very long time before it dies. In Halo 3, one melee will kill almost all Combat and Pure Forms, including the Brute and excluding Tank Forms. Melee attacks are useful against a Combat form when they jump and are preparing to land.
 * In Halo Combat Evolved, driving over a Carrier Form with a Warthog will not cause much damage to the player. However, in Halo 3, driving over a Carrier Form with a Warthog will flip the Warthog over because the Carrier Form's explosion will have the force of a grenade.
 * In Halo Combat Evolved, in the level 343 Guilty Spark, it is possible to see a few Combat Forms before going into the structure in the beginning of the level. You need a M6D Pistol to see them, though. They can be seen on the large tree stump that is far away from you.
 * 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)
 * It is possible to infect Sergeant Johnson into a Combat Form in the last level of Halo 3. (Kill him before you walk into the control room)
 * 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 or four Gravity Hammer hits. But encounters with these forms are rare.
 * In Halo 3, If the player has their back towards an Infection Form, occasionally it will grab the players head and drain their shields.
 * Even in large numbers, Infection Forms can be taken out with only a couple shots from the Assault Rifle as the weakness of the Infection forms, combined with the force of their explosion will "pop" nearby Infection Forms.
 * If a Flood Combat Form is neutralized but the Infection Form in it's chest is uninjured, the Infection form will remove itself from the original host to find a new one (seen in halo 2 and Halo 3).
 * 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.
 * In Halo 3, the human combat forms all have the same uniform, so if a Marine with specialist uniform gets infected, the uniform will change to the normal ones.
 * Similar to the above, if an Elite in Halo 3 gets infected (For example, a Major Domo), when he is destroyed, his armor will revert to the Blue of the Minor Domo. There are rare exceptions as sometimes, a Major Domo, when infected, will still have red armor.
 * The Sniper Rifle System 99C-S2 AM is very ineffective against the Flood due to their insides being liquefied. It usually takes about eight shots to kill a human Combat Form on Halo: Combat Evolved.
 * Human Flood can wield more weapons than Covenant Flood, the Covenants are compensated with a stronger melee attack, and some of them even still have working shields.
 * The Flood on Halo: Combat Evolved are extremely smaller than those of Halo 2 and Halo 3.
 * In ''Halo 2', Human Flood forms are much more formidable opponents than a Elite Flood form simply because they are harder to hit, faster, jump higher, and seem more aggressive.
 * In Halo 3, the Flood have a habit of calling you "The Iron Casket".
 * In Halo 2, Gravemind has control over 2401 Penitent Tangent allowing him to Teleport anywhere around the near by Installation 05, so he could have teleported away from The Ark and Installation 04 (II) in Halo 3 rendering him harmless.
 * If you look closely at the Flood Prophet form of Regret, you may see a Human skull lodged in its flesh.
 * One early theory of the Flood's origin was that they were a Forerunner experiment that went horribly wrong, but this idea has been disproven. It is more likely that the Flood had an extra-galactic origin.
 * It was unknown where the Flood came from, only that they are an extra-galactic species, ie: they come from another galaxy.
 * 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.
 * Flood Brain Forms/Proto-Graveminds are rarely seen in Halo:CE to Halo 3.
 * In the level 343 Guilty Spark, you see the Elite Flood combat forms near the trees. But when you use the Pelican trick and get to the part where you see the Flood on the large tree log, you can't see them anywhere. Nobody knows why they don't appear when you get up there.
 * It seems that the flood are weak to heat as some of the most effective weapons are heat-based(plasma weapons,flamethrowers,sentinel beams,etc....)as well as close quarter weapons.
 * On Flood levels in Halo: CE human Combat forms are rarely seen with fragmentation grenades when upon killed, although the Flood can not use or throw grenades.
 * If you turn on the catch skull pure forms drop spike grenades.
 * If you set a Flood on fire using the flamethrower or the incendiary grenade, it will occasionally run towards you, hoping to spread the fire to you or marines.
 * You can only have Flood buddies on Halo:CE.
 * The flood use every weapon in Halo CE except the sniper rifle.
 * 343 Guilty Spark is The first mission in the Halo series is when the UNSC First encounter the Flood.