The Flood is a parasitic alien life form driven by the need to infest sentient life it encounters in an effort to spread. Nearly 100 000 years ago, its spread forced the now extinct interstellar civilisation of the Forerunners to kill themselves and all other nearby sentient life in an effort to starve the Flood to death.The Flood is believed to be an engineered living weapon developed by the Precursor species as it was being eradicated by the Forerunners in a war that took place more than 150 000 years ago.
The most common form the Flood, dubbed “Infection form” can reproduce itself without the help of external biomass. It seeks to drive sharp spines in any nearby sentient lifeforme to tap into their nervous system and incapacitate them while it burrows in their body and begins the mutation process, bringing the host under Flood control. Hosts that have been recently converted are noted to be weaker than those who have been under Flood control for long periods.
Depending on the size or condition of the hosts, the Infection form turns them into into either "Combat forms" or “Carrier forms”. Combat Forms are extremely powerful and able to resist extreme punishment. They can use the host’s technology or the long, whip-like tentacles developed during their mutation to attack any non infected lifeform. The "Carrier Form" is created if a "Combat Form" is too damaged or ill-equipped to prove well in combat. Carrier forms hosts’s upper halves decome bubbley in appearence and carry around infection forms that are developing.
Once the flood’s absorption of sentient biomass has reached a certain critical level, it develops a centralized intelligence, called gravemind, from the bodies it has infected into a massive , tentacled intelligent entity resembling a large Venus Flytrap, capable of movement and linguistic communication which litteraly controls the flood. As a gravemind develops, it is able to exact more control over the lesser flood forms and is eventually able to develop entirely new forms referred to as Pure Forms. Pure forms do not require a host body to create and are instead formed from degraded biomatter. Pure forms are capable of limited shape shifting that allows them to quickly change their role within the flood.
Source: Halo 1, 2 & 3 Console games and related novels Halo: The Flood, Halo: First Strike