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.

Biology
The Flood is an anomaly among all life. The basic unit of the Flood is the Flood super cell, an exotic cellular structure capable of assimilating almost any type of nervous system to serve its needs, making any sentient species a potential vector. These super cells form 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. The Flood absorbs 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. 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 their presence altering the fabric of space-time on a local level, seemingly through sheer computing power alone. As of the modern era, such claims remain unsubstantiated but obviously frightening should they hold true.