The Flood is a fictional alien life form in the computer game Halo: Combat Evolved.

The Flood is a parasitic virulent life form which is spread by infected carrier hosts. The Flood appears in three primary forms: parasitic spores (Infection Forms) that attach themelves to hosts and transform the hosts's DNA, mutated hosts (Combat Forms) such as humans or Covenant, and Carrier Forms that explode, expelling spores that continue the process. Flood that manage to assimilate their hosts gain full control of the host body and access to the host's memories and brain functions, and tend to be very powerful and strong.

The origin of The Flood remains practically unknown. What is known is that the Forerunners constructed gas giant research facilities and later huge "Fortress World" Installations (called Halos by the Covenant) to contain and study The Flood. In the event that The Flood was released, the Halos were designed to destroy all sentient life in the galaxy. The thinking behind this is that if The Flood has no food then it will eventually starve to death, allowing the galaxy to slowly re-populate and preventing The Flood from spreading to even more galaxies throughout the Universe. In the sequel, Halo 2, some levels took place onboard a Forerunner facility. Throughout the levels were many small decorative pedestals supporting objects that are similiar in shape and colour to various forms of Flood. These decorations are repeated many times, too precisely to have been created by the Flood, and are probably intentional references to it.

In Halo 2 it is revealed by 343 Guilty Spark that there were originally seven Halos; six remain after one was destroyed in Halo 1. As each Halo has a firing range of 25,000 light years there are still enough Halos to destroy all sentient life in the galaxy.

The Flood were not completely destroyed in the first Halo game and made an unexpected return with the ability to use vehicles and command starships which could take them anywhere in the galaxy. Their Rulers are apparently Gravemind, who has a base of operations beneath Delta Halo's Library and on High Charity and its offshoot, a Command/Pilot/Brain form, formerly Captain Jacob Keyes who occupies the second Covenant cruiser in Halo: Combat Evolved . It is not currently known if there is a Gravemind in each Halo, or that the Gravemind is the controller of all Flood.

The name "The Flood" was most probably given due to the species' sheer force of numbers.