The Flood, or The Parasite, are a species of parasitic organisms that reproduce and grow by consuming sentient life forms. They consumed most sentient life in the galaxy during the Forerunner-Flood War, most notable the Forerunners, and are considered to be the greatest threat to the existence of life and biodiversity in the Milky Way

When the Forerunner-Precursor War drew to a close, several Precursors which were spared were pushed to the edges of the galaxy, some in a dust which could regenerate to their past forms. Over time the dust corrupted and mutated, and the Precursors embraced this, becoming the flood. They sent several ships of the mutated powder to Forerunner planets.

The Flood was not discovered by the Forerunners until after the Flood had already fled the galaxy when a cure was created. The Forerunners created the Halo Network as a precaution. Eventually, 10,000 years later, the Flood did return, and after many failed tactics the Forerunners were forced to activate the Halo Array, killing almost all of the flood as well as most other organisms in the galaxy. Many indexed species were then sent to planets to repopulate the galaxy.

The Flood are usually coordinated by a Gravemind, which is a combination of Flood Super Cells and the neuro-network of hundreds or thousands of sentient beings. The way it controls the flood is similar to how humans control limbs. During the feral stage, without a Gravemind, flood infect as many beings as possible, particularly intelligent ones, as this speeds the creation of a Gravemind. Once a Gravemind is created, the Coordinated Stage begins, where the flood is controlled by a nearby Gravemind.

The Flood are featured in the campaign of Halo 1, 2, 3, Halo Wars, and Halo Spartan Assault. In Halo 4, there is a multiplayer gametype called Flood in which a growing number of player-controlled Flood try to eliminate player-controlled Spartans.