The Flood
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 was responsible for consuming most of the sentient life in the galaxy, notably 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 Sangheili, and Jiralhanae, 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 were an extremely adaptable, dangerous, and expendable parasitic life form.
They were first encountered in the Milky Way galaxy by an Ancient Human empire 10,000 years prior to the Forerunner-Flood war. The earliest known instance of the highly infectious viral super cell later known as The Flood form to be encountered came in a fine desiccated powder, contained in cylinders.
Early experiments showed the potential for psychotropic effects in some lower levels but not in humans or San'Shyuum. The primary animals affected were popular pets in human societies: the Pheru, lively and gentle creatures found on Faun Hakkor.
Very small quantities of the powder induced changes in the Pheru that improved their domestic behavior. Soon San'Shyuum adopted the custom of taking Pherus as pets. For centuries humans and San'Shyuum bred and powdered these animals without ill consequences. However the powder soon attached itself to key points in the genes of the Pheru and began to change them, which manifested itself as a peculiar growth found on a third of the powdered Pheru.
A kind of loose, soft fur grew between the shoulders of the pets. Other Pheru were soon found grazing on these companions, consuming their fur-and on occasion even consuming the animals themselves. Pheru were naturally herbivores. This seemed to activate some sort of biological timer, a signal for expansion. Within a very short time, the Pheru were producing far less attractive growths. Flexible striped rods sprouted from their heads, which in turn were also consumed by fellow Pheru, causing abortions and unnatural births.
But this was only the surface of the growing infestation. Eventually the Flood evolved into the form that we know today and fought a war with both the Ancient Human Empire and the San'Shyuum.
Unknown to the Forerunners, the Human/San'Shyuum alliance had developed a way to combat and eventually defeat the flood.
By manipulating key strands of the Floods genetic buildup, infecting a third of the human population, then placing them directly along the pathway of the Flood, the humans were able to 'reprogram' the intentions of the Flood.
Rather than wanting to consume sentient life, the Flood turned on itself, seeing other members of the mutation as being enemies.
As a result, the Flood began to tear itself apart on a galactic level further spreading the Human/San'shyuum genetic weapon, and causing the Flood to simply wipe itself out.
Knowing that it faced its own complete extinction, what remained of the uninfected, stable specimens fled the galaxy aboard a commandeered vessel, destination unknown and remained dormant for thousands of years, biding it's time.
The Forerunners learned of the peril and as a precaution designed and subsequently constructed the 12 individual Halos should the threat one day return.
As expected, 10,000(or 9,700 years, for it was said it destroyed 12 sytems of forunners in 300 years which ends the 10k years) years later, the Flood returned and began its conquest of total galactic domination.
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, effectively removing both the source and its entire food supply.
The Flood exists in a fairly stable state in the absence of a Gravemind. They lack the ability to coordinate their efforts, but the Flood will 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. Therefore, 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, the Gravemind.
Although little 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 a "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".