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

The Flood are extremely adaptable, dangerous, and expendable parasitic lifeforms. They were first encountered in the Milky Way Galaxy by a Forerunner pioneer group on planet G617 g1. It is a highly infectious viral supercell, and the cause of the "Forerunner-Flood War", which lasted approximately 300 years. They were the reason that the Halo Array was constructed, as a last final effort by the Forerunners to preserve biological diversity in the galaxy. After exhausting every other strategic option, the Halo Array was fired, resulting in the elimination of the original source of the Flood infestation and the death of every sentient being in the Galaxy.

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

During its Feral Stage, the Flood is only capable of local coordination through use of pheromone-based communication. Research indicates that the content of said messages are of very limited complexity. Individual Combat and Carrier Forms have access to the skills and memories of their host; however, once an outbreak establishes a viable Gravemind, the Coordinated Stage begins—and it is at this point the Flood becomes truly dangerous. With the formation of a Gravemind, the assimilated intelligence and memories of consumed hosts become part of a collective consciousness, which is capable of strategic thinking and commanding individual Flood forms in an organized manner.

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

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

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

The Flood are believed to have arrived in the Milky Way Galaxy from an unknown galaxy, more than 100,000 years prior to 2552 when they encountered the Forerunners on the planet G617 g1. Initially, the Forerunners severely underestimated the potential threat this new life-form posed, and used tactics more suited to disease outbreak than actual warfare. By the time the infection had begun to spread, the Flood had become a much deadlier and more intelligent foe. The Flood used unarmed civilian assets to penetrate planetary defense groups, first striking at the Forerunner-held planet of LP 656-38 e, bypassing the Forerunner Orbital Fleet, and infesting the planet below. They soon moved to similarly infest the planet DM-3-1123b. Forerunner ground forces were overcome by the sheer numbers of Flood forms on the planets. While at first the Flood demonstrated a lack of even "basic cohesion", they had "numerical superiority"; billions of forms dedicated to the assault, and every member of the Forerunner population a potential Flood host. In desperation, the Forerunner's armada was ordered to immediately commence full planetary bombardment on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before bombardment commenced, leaving the Forerunner military with shallow victories against the Flood. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers appeared to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.

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

While the Flood were exponentially growing, spreading from system to system with the intent to infect the Forerunner population, not to wage war with Forerunner battle groups, the tide was turning: the Flood had reached sufficient proportions to create a Gravemind form, a creature that embodied a collective sentient intelligence for the Flood species, and one that could coordinate the Flood swarms in intelligent attacks against the Forerunner Fleets. The Forerunners were aware of this, and constructed the Contender class artificial intelligence Mendicant Bias in an effort to destroy this centralized Gravemind and disorganize the Flood. This too failed since Mendicant Bias went rampant and joined the Flood cause to destroy its former masters. The Forerunners began a "Conservation Measure", researching the Flood, while constructing an Artifact to generate a slipspace portal leading to the Ark on Earth and possibly other uninfected home worlds of Tier 7 races they considered "worthy".

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

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

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

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

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

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

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

Infection forms selectively target other species that possess sentient intelligence and are of sufficient biomass, and can infest living or lightly wounded dead bodies. Infection forms have long tendrils that pierce the skin of the host and find their way to the spinal cord. From here they synchronize with the host, matching frequencies with the host's nervous system and taking over brain functions, completely rewriting the psychology of the creature to the ravenous and voracious psyche of The Flood, although retaining higher-level cognitive abilities, such as combat techniques and technical knowledge which may be useful. However, resistance of the host is completely destroyed, as the original mind's memories are systematically erased, leaving only a simplistic and primal urge to assimilate other species and destroy them. During the early stages of infestation, when the host still retains its original consciousness, there are distinct somatosensory sensations: first the host's extremities grow numb and their lungs feel heavy.

Then, the greater Flood intelligence begins to take over the mind, and sensation becomes increasingly indistinct: the host's vision seems to be "a gauzy white haze", but the host can make out shapes and the host can hear a low droning through the ordeal. Finally, once physiological mutilation occurs, the host can vaguely feel dramatic changes to its own body: the limbs feel to the host as if they are made of a spongy liquid and the lungs itch as well. However, the host can still control their limbs to some extent if the neurological contact with the infection form is weakened. They can use the vocal cords of Jiralhanae's, Humans, and Sangheili's to communicate as well as the moans they use as observed on The Covenant where the drop pod crashes into the third shield tower.

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

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

Higher-level species capable of combat, such as Humans, Jiralhanae, or Sangheili are selectively infected to become Combat Forms. Combat Forms retain the general profile of their original species, although they are significantly mutated. Organ-based systems' physiology are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible; organ functions are decentralized, thus, decapitation does not down a combat form. Furthermore, highly area-specific munitions, such as sniper rifle projectiles, are completely ineffective, as picking off selective areas of a combat form leads to no wide-scale physiological impediments, and the projectile will punch through the decayed flesh rapidly, exiting through the other side of the combat form without consequence.

Combat forms also generate tentacles very quickly without regard for the skeletal structure of the host, with tentacles protruding from the combat form at odd locations, typically localized to the chest axial regions, such as the neck, armpits, or torso. A single arm is oftentimes mutated to a claw like or tentacle-like structure that splits into multiple angular, hand-like structures replacing the hand's usage, yet it will still be there. The musculature of the combat form dramatically changes as well, and the form is given prodigious arm and leg strength, allowing for heavy blows with its arm-like structures and for incredible speed and jumping height with its legs. The Flood are also prodigious climbers, able to scamper on walls with little hindrance to the organism . Metabolic requirements, however, appear to be extreme on the organism: blood vessels become extremely prominent on the surface of the Combat Form, with thick and distinct blood-colored vessels spawning all across the creature's surface, most prominently so on the head.