Flood

The Flood, categorized as LF.Xx.3273 by the Forerunners and commonly known as the Parasite by the Covenant, is an extremely virulent omni-parasitic lifeform.

History
Suspected to be extragalactic in origin, the Flood was first encountered by a Forerunner pioneer group on G 617 g, a previously-uncharted planet at the fringes of the Milky Way galaxy. After a conflict lasting three centuries, the Forerunners, unable to control the now galaxy-wide outbreak via conventional means, activated their weapon of last resort, the Halo Array in 97,445 BCE. The Array sterilized the galaxy of sentient life, reducing the Flood to its primitive cellular form and effectively starving it to death. A number of samples of Flood super cells and infection forms were held in stasis within select Forerunner containment facilities, however, with the intent of discovering a cure or inoculation to the parasite. This proved a mistake, as the Flood broke containment again on several of these facilities, though these outbreaks remained largely localized until the year 2552. Battles between the UNSC and the Covenant led to their release on Installation 04 and soon after Installation 05, where the Gravemind that had formed on the ring eventually infiltrated and took over the Covenant holy city, High Charity. However, the Gravemind was defeated soon after in the Battle of the Ark, where the firing of the incomplete Installation 08 sterilized the local Flood outbreak. Coupled with the destruction of plagueships dispersed by High Charity beforehand, this seemingly marked the end of the Flood threat; however, some suspected that pockets that eluded the Schismatic fleet's quarantine may have survived in unknown regions of the galaxy.

After the war, the human public is aware of the Flood on a basic level. Although the Office of Naval Intelligence was at first intent on covering up the parasite, the extent of the Flood outbreak in Voi made it impossible to cover up the subsequent precautions taken to ensure no spores survived in the atmosphere in the wake of the Fleet of Retribution's glassing of the region. To the general public, the Flood is framed as a dangerous alien virus which, while alarming, is not the first incidence of such in human history (the colony world of Barugi, for example, was quarantined indefinitely due to an uncontrollable pandemic). While citizens are instructed to recognize telltale signs of Flood infection and report said signs to the authorities, the full horror of the Flood is downplayed in order to prevent panic (for example, instances of the parasite reanimating the dead are dismissed as "anecdotal"). The UNSC's various precautions involving potential Flood residue, particularly in East Africa, have also been cause to controversy. There are some who maintain that the Flood is a fabrication and part of a government conspiracy and an excuse for the UNSC to hold onto its power after the war.

Biology
An anomaly among all known life, the Flood has been described by some (xenobiologists included) as unlife. It is most definitely not biological in any meaningful sense of the word, nor is it likely to have evolved naturally. The most basic unit of the Flood is the Flood super cell, an exotic cellular-scale structure capable of assimilating almost any type of nervous system to serve its needs, making any sentient species a potential vector. After an outbreak reaches a certain threshold, nonsentient biomass as well as inorganic matter can be co-opted. The super cell is hypothesized to be a form of self-replicating pico- or femtomachine — behaving in ways analogous to a life-form in many ways, but almost certainly artificial.

These super cells comprise Flood spores, which in turn are often carried by specialized infection forms created to physically seek out and assault potential hosts. Upon an outbreak, the Flood converts its hosts into a number of discrete forms, with roles ranging from combat to the incubation of new infection forms. One of the Flood's core mechanisms is its preservation of complex information patterns, such as individual memories and mental states, but it also twists them into a horrific mockery. Through this process, the Flood is able to absorb the memories and knowledge of its victims, thereby growing in its cognitive capacity with each new host. While relatively feral and uncoordinated in the early stages of an outbreak, the Flood begins to instinctively gather biomass to form a Gravemind, a powerful collective intelligence that emerges when a Flood outbreak reaches critical mass.

Psychology and behavior
A form of biological hegemonizing swarm, the sole goal of the Flood superorganism appears to be to spread by converting any life it encounters into more of itself. It is unknown, and perhaps unknowable, whether the Flood is a form of biomimic machinery fulfilling directives as twisted as they are ancient, or if the directives governing its behavior have themselves mutated uncontrollably with time in a process not unlike biological evolution.

When fully formed, the Gravemind seeks to hijack any available channels of communication and saturate said channels with noise, often using the voices of its victims to mock, confound and demoralize its foes as a form of psychological warfare. Through its vast intellect and computing power, the Flood is even capable of overwhelming computer networks with weaponized code strings and even converting AIs to its cause, the most notorious example of this being the Forerunner military AI Mendicant Bias. While the exact mechanisms of this remain unknown, it is evident that the Gravemind effectively inhabits and controls all Flood forms over great distances. There are reports of Gravemind forms during the Forerunner era reaching a level of cognitive ability described as "transsentience", with claims of Flood nodes breaking the known laws of physics and altering the fabric of space-time on a local level, seemingly through sheer computing power or some other, unknowable mechanism. As of the modern era, such claims remain unsubstantiated but obviously frightening should they hold true. It is apparent that, when a Gravemind reaches a sufficient state of development, it is able to access the knowledge of past Graveminds going back to the Diluvial War and perhaps even predating it, though it should be noted that this is based on statements by the Gravemind itself, which may or may not be a reliable source of information.

The nature of the psychology of the Flood is conjectural, and is likely to remain that way. It is known that a sufficiently large outbreak of Flood develops a central coordinating mind, but it is not known if that intellect is a hive mind or a unitary intelligence, or if the Gravemind has conventional instincts and motivations that we can understand. One purely instinctual behavior has been observed in the outbreaks on Alpha Halo and High Charity, and quite possibly on the shield world Etran Harborage: when suddenly threatened, a Gravemind will call in all forces under its control, drowning attackers in waves upon waves of combat forms. The Forerunner themselves noticed this behavior, and likened it to a panicked fighter throwing up his arms to protect his head. Unfortunately, the Forerunner were rarely in a position to provoke this behavior in the latter stages of the war.