The Flood

From Halo.wikia.com:

The Flood (Latin Inferi redivivus, meaning "renovated dead") also known as the Parasite by the Covenant, is a species of virulent, savage, parasitic organisms that thrive by consuming all sentient lifeforms of sufficient biomass and cognitive capability.

Their origins are currently unknown, but it is known that they are an extragalactic species bent on consuming all sentient life throughout the entire universe. In regards to every outbreak of the Flood, there exists three or four stages the parasitic organism undergoes, which are feral, coordinated and interstellar with the possibility of another higher stage present and intergalactic. In the feral stage, the Flood make use of the four basic forms: Infection, Combat, Carrier and a proto-Gravemind form (Brain Form). Once a viable Gravemind has developed, the Coordinated stage begins with the numbers of forms growing rapidly. The Interstellar stage develops once the Flood have acquired sufficient technology and do not possess any further known evolutionary changes. The Intergalactic stage is the last known stage of the Flood, once it has consumed all sentient life in a galaxy (and by extension, gained complete understanding of any technological and scientific knowledge that was present), it will have reached a complete critical mass, which must depart to other galaxies, possibly by either entering a dormant state or by utilizing said gained knowledge and technology, for further replication. In retrospect, the Flood follows a systematic flow of replication similar to the lysogenic and lytic cycles of viruses, but on a universal scale as opposed to a cellular scale.