The Flood, referred to as the Parasite by the Covenant, is 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 sentient life in the galaxy, including many Forerunners, during the 300-year-long Forerunner-Flood War.
The Flood presents a highly variable and unconventional threat in combat, as it can infect and mutate dead or captured opponents into a myriad of deadly forms, effectively growing stronger as it weakens enemy forces. They are widely considered to be the greatest threat to the existence of life (or more accurately biodiversity) in the Milky Way galaxy.
- Feral Stage
- In their feral stage, the Flood are at their simplistic form. They take on the most basic forms of alteration. In this stage, the Flood are exacting their goal of unification. They start by attacking any and all sentient life around, even using their own weapons against them. While attacking, the Flood take no formation and attack like brutes, head on. Their goal in this stage is to create a Gravemind capable of coordinating their attacks increasing their efficiency. To create the Gravemind, the Flood form a mass compound known as the Proto-Gravemind.
- Coordinated Stage
- In this stage, the Flood become increasingly dangerous. They are strategically coordinated by single or multiple Graveminds.
- Interstellar Stage
- The Flood are capable of consuming an entire solar system. They have successfully consumed their first planets and attained space-faring technology which allows them to move on to other worlds.
- Intergalactic Stage
- In this stage, the Flood have spread out throughout most of a galaxy. Entire systems had fallen victim to the Flood and victory seems almost inevitable. By this stage, the best corse of action to take against the Flood is the Halo Array. There are other ways such as using Shield Worlds or creating the Maginot Line, but both proved to be failures against this stage of Flood. It's primary function: killing all sentient life in the galaxy, thus starving the Flood to defeat.
The Cure Effort
In November 17, 2552, a Flood-infested battlecruiser crash landed in Africa. The infection was halted, but samples were recovered by the Office of Naval Intelligence. Construction of a secret research facility on Asteroid Colony N4-19 would begin in 2553 and be completed later that year. Beta-4 Division recruited highly qualified experts in the fields of genetics, biology, chemistry, pharmacology, and virology. Though most of these researchers would come from branches in the Manticore Consolidated Sciences, others would stem from other research institutes across the colonies such as TRI International.
The Flood uses horizontal horizontal gene transfer which transfers genes from the bacterium into other organisms, namely sentient beings. Unlike vertical gene transfer where genes are transferred from parent to offspring, horizontal transfers the genes between species and this transfer occurs at an unparalleled rate. The super cell spreads throughout the host at near-instantaneous speeds which means isolating or remotely targeting the infection is near-impossible. The parasitic cell feeds off of mostly calcium reserves and protein, and then augments the host's body. The scientists have tried experimenting with synthetic organic matter, a form of bait that could then be "poisoned."
In 2554, scientists fed this synthetic organic matter to infected hosts in an attempt to condition the parasite to dislike the taste of sentient beings. The experiment was a failure as the Flood proved more versatile in separating the calcium and protein from the "poison" and ignoring the poison. This process was done on an atomic level. Another experiment was attempted at replicating the desirable taste with something lethal to the parasite, but the infected host showed no desire for it.
Genetic engineering has no effect on the super cell as it is self-repairing. Scientists have tried to engineer the genome of the cell in an effort to guide it's functions, but there's an unknown agent inside the nucleus that repairs the genome. It has been described as almost intelligent, a species of its own and likely the source of the Gravemind's intelligence. The elusive agent has yet to be understood and it is questionable as to whether it is even physical or ethereal. Quantum Physicist Darwin Lavigne speculates that this unknown agent is not of biological origin but possibly quantum mechanics.
It wasn't until in the later 2550s that he developed the theory that these agents were static virtual particles that didn't or probably are being kept from blinking out of existence. The concept resonates with the Precursor's idea that the Universe is living. This would also explain the Gravemind's unique connection with space, to the point that he can even manipulate the filament's that bind space together. Though it is difficult to explain, the Domain has revealed some evidence that aligns with Lavigne's theories.