Flood

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

- Gravemind

"Some kind of hostile - isn't Covenant."

- Victor 933

The Flood (Latin Inferi redivivus) also known as the Parasite by the Covenant, is a species of virulent, savage, parasitic organisms that thrive by consuming sentient lifeforms of sufficient biomass and cognitive capability.

Introduction and History
It would seem that The Flood is the most powerful species in the universe. Some forms display sentient behavior, and are parasitic. They prey on hosts that have some form of sentience and enough mass. How the different types of Flood actually work is not known. But when they take over a host the Flood is extremely adept at using technology, weaponry, and machinery of the host they’ve taken over. When the Forerunners fired the Halos around 100,000 years ago it was supposed to eradicate all sentient life in the Milky Way galaxy. But the Flood's continued existence suggests that they are more powerful than even the Forerunners knew. It’s possible they survived through the more base forms that appear to not have any sentience by themselves. (In Halo:Combat Evolved, Cortana states, "...Halo doesn't kill Flood; it kills their food!")

Their origins are currently unknown, although speculation is that the Precursors created the Flood to lead the Forerunners into the discovery of Humanity, though this is unlikely because the Precursors left the "Mantle" to the Forerunners, and killing them off would not make much sense. Also speculated is that perhaps the Precursors created the Flood as a means of biological war with another species, and the Flood became self aware, and eventually spread to other galaxies, though this is also unlikely. The most complete theory is that the Flood was simply an extra-galactic virus that somehow became genetically mutated, causing the infectious vectors to grow from cellular to biological, or perhaps planetary as seen on High Charity.

In regards to every outbreak of the Flood, there exists numerous stages the parasitic organism undergoes which are feral, coordinated and interstellar with the possibility of another higher stage present, intergalactic. In the feral stage, the Flood make use of the four basic forms: Infection, Combat, Carrier and a proto-Gravemind form (Brain Form). Once a viable Gravemind has developed, the Coordinated stage begins with the numbers of forms growing rapidly. The Interstellar stage develops once the Flood have acquired sufficient technology and do not possess any further known evolutionary changes. The Intergalactic stage is the last known stage of the Flood, once it has consumed all sentient life in a galaxy (and by extension, gained complete understanding of any technological and scientific knowledge that was present), it has reached a complete critical mass, which must depart to other galaxies (possibly by either entering a dormant state or by utilizing said gained knowledge and technology) for further replication. In retrospect, the Flood follows a systematic flow of replication similar to the lysogenic and lytic cycles of viruses, but on a universal scale as opposed to cellular.

The Flood's potential for destruction is not in doubt. It is an organism-scale parasitic species, and its sole means of reproduction is by assimilating other sentient species into its fold by neurological and physiological mutagenic means by entering and infesting live bodies or corpses. Its all-consuming nature and dependence on sentient life means that it would be impossible for the Flood to be a part of any ecosystem.

Forerunner-Flood War
The flood are believed to have arrived 100,000 years prior to 2552 at 98,379 BC, when they encountered the Forerunners. The flood used unarmed civilian assests to penetrate planetary defense groups, first attacking 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-3-1123 b. 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 infestation of each Forerunner world. Forerunner fleets were ordered to commence full planetary bombardment on infested worlds, although at 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, destroying themselves before the Flood could infest them in "noble sacrifice[s]".

Thereafter, the Forerunners realized that naval tactics were unable to stem the growing tide of the Flood, and began to invest in superweaponry. While the Forerunners initially deployed the Sentinel electromechanical drones to fight against and contain the Flood, almost immediately afterwards, the Forerunner Fleet Command contemplated "premature stellar collapse": using naval battlegroups to destroy planetary star systems' primary stars in supernovae that would engulf the planetary systems and prevent any possibility of Flood infection.

While the Flood were practicing "exponential growth", spreading from system to system with the intent to infest the Forerunner populations, not to wage war with Forerunner battlegroups, 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-classification 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 the artifact that would create a portal to the Ark on Earth, protecting the "worthy" human race from destruction from the Flood. Some Flood specimens were kept by the Forerunners for scientific study.

The Forerunners employed many, many measures, including intricate naval tactics, in an attempt to contain the Flood, but ultimately failed. Finally, the Forerunners were forced to a Pyrrhic solution, building seven ring-like installations, known as The 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 lifeforms in the galaxy, would starve the Flood out. After all other measures failed, the Forerunners activated the Halos in 97,448 B.C., unleashing galactic destruction, destroying almost all sentient lifeforms. However, the human race and all other species, on Earth, were shielded by the firing of the Halos by the Ark and its Portal.

The Flood were contained, lacking hosts and thus unable to reproduce. The only surviving specimens were contained in top-class, high security Forerunner research facilities, such as the Halo installations and the Threshold Gas Mine.

Release
Although the Flood were temporarily contained by the first firing of the Halos, they eventually rose again. The Monitor states to the Master Chief that, "10,000 years ago, the alien species you know unleashed the Flood upon this ring. Though the contamination was small, and died out once it destroyed its food, the Flood snaked throughout the tunnels under the surface of Halo." That is how the Flood got to the structure in the swamp where the Covenant find them the second time, and where, subsequently, Keyes is killed. Another Gravemind creature was spawned, directing the Flood's efforts, although initially, the Flood remained dormant, merely kept in Forerunner research facilities, carefully guarded by the Forerunner Sentinel drones. However, in 2552, the Flood were unleashed and began to exponentially multiply. When the Covenant stumbled upon Installation 04, "Alpha Halo", the Covenant began massive excavation projects and exploration of Forerunner facilities, believed to have been attempting to recover Forerunner artifacts. In the ensuing Battle of Installation 04, the Covenant discovered the Flood, and interfered with the Forerunner defenses keeping them in check.

A garrison of Covenant forces, including Elites, Jackals, and Grunts, moved into an underground Forerunner installation in the middle of a foggy swamp, converting it into a Covenant arms cache, but their explorations eventually unlocked rooms where the Flood were being held, thereby releasing them.

They were released once more by the humans, believing they were headed into a weapons cache. When they unlocked the door, they were swarmed by thousands of infection forms.

With a large source of vulnerable hosts in the form of the Covenant and the humans, the Flood began to exponentially reproduce again, and within hours, Forerunner facilities on Installation 04 were overrun as the Flood, reinvigorated with a new source of hosts, overwhelmed the natural security measures and began to destroy the Sentinel custodians on Installation 04, branching out to infest the surface and perpetuating to infiltrate the Sentinel-guarded underground Forerunner facilities, overwhelming the Sentinels even underground and seizing control of most underground installations.

Assault
At the time, Covenant forces, widely stretched across Installation 04, were massively engaged and 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, Grunts and Jackals crumpled easily to the Flood, and the smaller number of Elites were being outnumbered and destroyed. The UNSC, having a much smaller presence, found out about the Flood on a widespread scale days later, when a UNSC Orbital Drop Shock Trooper force engaged with the Covenant, nearby Alpha Base was ambushed by the Flood, with bitterly high 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 infestation had been dulled by years of hibernation, making Jenkins's conversion into a combat form incomplete. Jenkins still held mental resistance against the pervasive Flood mind-being 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 companies that the Flood planned to attack the UNSC base from underground. Jenkins was later captured for a source of intelligence against the Flood threat, and provided intelligence that the Flood planned to assault the UNSC base from a series of underground catacombs underneath the base, and the UNSC moved to secure them, thus preventing the Flood from staging another attack.

By that time, Flood dominance over the Covenant was being assured. The Sentinels were extremely rare, and unable to contain the Flood. Meanwhile, whole rooms formerly held by the Covenant were already completely taken by the Flood, although Covenant and Flood forces continued to wage war on massive ice plains, with the Covenant having the benefit of Shade stationary plasma turrets and vehicular/aerial support and the Flood sporting M19 Rocket Launchers and massive numbers.

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 infesting infection form, while deleting his memories, felt that Keyes held vital memories...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 synthetically merged with at least two other human hosts 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 arrivel was too late, for Keyes was already dead. It was at that time, the Master Chief punched a hole through Keys skull too get the Captain's CNI.

The Truth and Reconciliation was a major point of contestation between the Covenant and the Flood. The Flood appeared to have entered the warship previously, decimating most of the crew and garrison, leaving isolated pockets of resistance, mostly of 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 against 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.

Defeat
The Flood appeared to be interested in seizing space-capable vessels in the hope of spreading the Flood away from the Halo rings and into the stars: they also attacked the UNSC Pillar of Autumn. After the UNSC cruiser's crash-landing onto the ring after engaging over fifteen enemy warships, it had been seized 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 Elites-turned-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 were unable to stop SPARTAN-117 from detonating the wrecked warship's reactors and escaping while the thermonuclear detonation severed the ring world, destroying its capability to annihilate all sentient life, and killing all remaining Flood on Installation 04.

However, this did not mark the end of the Flood, who were still widely disseminated across the galaxy.

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 kill Sesa 'Refumee, a Heretic Elite stationed within the mine that was encouraging dissent against the Prophets. While the Covenant Special Operations force attempted to track down and assassinate 'Refumee, 'Refumee, by accident or design, unleashed the Flood that were kept within the station.

During the Arbiter's reckless pursuit of 'Refumee, 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 deliberately 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 the Flood while Heretics dueled the Flood underneath them. When that trap failed to annihilate them, '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" as 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 had cleverly laid another trap, locking the doors while Flood forms poured in. 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 crushed while a horrid tide of infection forms, combat forms, and carrier forms lurched into the hall as the Arbiter was forced to defend against them until the tide stilled and the doors were opened.

'Refumee's plan of unleashing the Flood upon the pursuing Covenant Special Operations strike force, however, was not nearly a precise plan. The Flood would not be contained into rooms and merely attack the Covenant strike force. 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 forced to pursue 'Refumee, but was heavily impended by large waves of Flood crushing outnumbered 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 destroyed the entire Gas Mine, destabilizing it. The station was sucked into a maelstrom in the lower atmospheric depths of Threshold, very likely crushing all Flood and Heretics still aboard.

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, the Flood had been in continual war after 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 fast rate: within hours, Sentinel forces were being overrun.

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, massive, shielded aerial vehicles that were extremely effective against vehicles and infantry alike.

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 "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 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 dropship, 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, there were 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 an LZ 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.

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 and The Arbiter to High Charity and 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 Human 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 Drop-ships 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 being 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 Forerunner Ship that was leading the Covenant 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 Human knowledge from her.

Earth, The Ark, And The Flood's Defeat
''"Child of my enemy, why have you come?! I offer no forgiveness for fathers' sins, cast to his son."

- Gravemind on the level Cortana '' During the Battle of Voi, an infested Covenant cruiser crashed into the city of Voi. Miranda Keyes ordered John-117 to detonate the infested cruiser's reactors and search the ship for Cortana, causing the city of Voi to be destroyed, and the Flood eliminated. The Master Chief, with the help of the Elites, was able to find a message from Cortana aboard the cruiser, while the Elites went on to glass the infected areas. The forces, fulfilling their mission of removing the Flood from Earth, retreated along with the message.

On the Ark, while attempting to lower the shield surrounding the Ark's citadel, the infested High Charity made a slip space jump and crashed into the Ark, releasing the Flood inside the structure and causing an infestation on the newly built replacement for the destroyed Installation 04. SPARTAN 117 was able to kill all Flood present on the Ark and the replacement Installation 04 by activating the unfinished 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.

Aftermath
With the Gravemind destroyed and all the Flood on the Ark gone, it is possible that the Flood are now extinct. However, it is also possible that the Flood still exist on the other Halo rings (excluding Installations 04 and 05 although it is possible that all of the Flood on 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 anyway further complicates the issue. For now the Flood are seemingly defeated.

Biology
A Marine is infected by the Flood (Double-click for larger video) N-bCMhBINiA 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 sole known pathway for Flood reproduction and survival is by the infestation and assimilation of other species.

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 animalistic hunger 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 level "The Covenant" where the asteroid 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"(Rarely). It has been said that it is the Gravemind communicating to you using the infected beings.

Physiological
While baseline neurological assimilation is 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 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 is corrupted, and organ-specific functions are decentralized and body cavities are decayed, making hit box-selective incapacitation impossible: since organ functions are decentralized, 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 clawlike 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 millennia of Flood existence, it is believed that the Flood have evolved extensively so that there are few direct countermeasures against infection.

There is no confirmed way to effectively stop a Flood infection when a host is infected 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.

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 nervous system, completely changing its psychology to that of the Flood's, and eventually physiologically mutating it into one of the various Flood forms.

Infection Form


The infection form is the most basic and simple Flood form, and is the smallest and not to mention 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 vile creatures attack in dozens or even hundreds. Their discovery on Halo indicated that they can remain dormant for countless millennia until a suitable host wakens them. 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. However, energy shielding provides some protecting against an infection form, popping the creature in a small, but damaging, blast of noxious gas.

After infecting combat forms, 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! 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.

Combat Form


After an infection form has successfully latched onto 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 infestation. Infection forms have been seen only mutating humans, Brutes and Elites, not bothering to utilize Grunts or Jackals presumably because of their physical weakness and lack of significant 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 most damaging 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 circuity. 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 a Brute, Elite, Marine, or SPARTAN.

As mentioned below, combat forms can employ 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. Combat forms are rakishly aggressive and lack even rudimentary battlefield tactics, although their sheer numbers can force even powerful enemies to succumb. Infected Elites 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 burn infected corpses to ensure that they can not be reanimated.

They can sometimes be heard talking in Halo 3. It is apparently the Gravemind speaking through them, as all flood in an area speak with an echoing near simultaneity, in the same tone, speaking the same sentence. They use taunts such as "I will be your doom", and calling the player's armor a "casket".

Carrier Form


The exact origin of Carrier Forms is unknown, although they are theorized to be combat forms 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.

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, forcing the carrier form 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 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.

In comparison to both Halo 1 and Halo 2 ,Halo 3's Carrier Form is much more difficult to destroy.

Pure Form



 * Main Article: Pure Form

A new 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 turn into tank forms,tank forms maybe mistaken for a Hunter form because a tank form has one oversized hand (equivalent to a Hunter's shield) and a small arm (equivalent to where it's gun once was) and even more similar size but with research you will find out tank forms are not infected Hunters

Prophet Form


Prophet Forms are highly rare products of infestation and assimilation. 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 kraken-like 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.

Proto-Gravemind
One of the rarest types of Flood is the Proto-Gravemind. This form is unlike all other Flood forms since it lacks any sort of mobility and is passive. These are 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 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. 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 taken by the flood in combat form originally, and was then merged with the Proto-Gravemind on the bridge of the CCS-class battlecruiser, the Truth and Reconciliation.

Gravemind


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 Proto-Gravemind but on a larger scale, 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, through extraordinary and enigmatic "genetic" means. While it is capable of directing the overall Flood species, it also possess 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 laired 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.

Aquatic
The aquatic form is never encountered in combat and its use by the flood is unknown. They were on the level Oracle inside a small glass and metal tank. They are un-killable and don't react to anything. Whether they are a natural flood form or a Forerunner experiment is not clear. The only indication that they are alive is that they swim in a school in circles around the tank. It is possible that they could be flood-infected fish. It could be an alien form of fish, making it unrecognizable.

Combat Behavior
While possessing an unusual sentient intelligence and 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, in their psychology 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, oftentimes 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's MJOLNIR Armor's shields in half with a single blow. 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 then 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, and the Flood were gathering body parts for unknown purpose 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. In addition, the Flood can instantly absorb knowledge from an infected host into their collective mind.

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 mutilated. At times, the mutations can occur randomly and uniformly: according to the Halo Graphic Novel, and 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 appears to have 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.

Tactics
It is best to use the Assault Rifle/Shotgun against the Flood in Halo 1. In Halo 2, dual-SMGs has proven to be very useful, as well as the Shotgun. However, all other weapons except heavy weapons and Sentinel Beams are useless. In Halo 3, The Shotgun, Energy Sword, Assault Rifle and Needler are all great against the Flood, as well as heavy weapons. Dual-wielding Plasma Rifles and aiming at the heads of the Flood is more than enough to rip them apart easily. The Assault Rifle is excellent for combat forms and infection forms alike, though it is unfortunatly uneffective against pure forms. The SMG is also as good against the Flood as Halo 2, give or take a couple bullets. Also effective is the Battle Riffle, as aiming for the infection form in the chest of the combat forms will kill the infection form controlling the Combat form. However, this tactic is rather ineffective against the pure forms, as they are not controlled by an infection form and thus cannot be "decapitated". Energy swords are by far the best weapons to use against pure forms, as even on legendary, they will still kill a pure form tank with one hit. Gravity hammers are good for large groups of flood, but aren't as effective against pure forms as energy swords.

Trivia

 * The first known human to be infected by the Flood was a UNSC Marine, 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.
 * 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 "How To Make Flood Friends", "Harmless when Armless", or "How To Make A Flood Buddy", although occasionally, the Flood form would attack with invisible arms. This was disabled in Halo 2 and Halo 3.
 * 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 made of Brutes, Elites or Humans 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.
 * In Halo 3, The Flood (or Gravemind) often call the Master Chief's armor his coffin, Iron Coffin, or The Iron Casket.
 * There may be more Flood in the other parts of the Milky Way on other Installations or Research Facilities but are probably secured by other Forerunner Constructs, preventing them from breaking loose.
 * 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.
 * The Flood's name has several potential origins. The one, mentioned in the books, is the one, that they "break over you like a flood". Another one as a biblical reference to Noah's Arc that rescued two animals of every kind from the Flood.