The Flood, designated as LF.Xx.3273 by the Forerunners (Latin Inferi redivivus meaning "the dead reincarnated") referred to as the Parasite and the infection by the Covenant, the Sickness and Necromorphs by humans, and Species 6 by the Borg, is a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient lifeforms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient lifeforms in the Milky Way galaxy - including the vast majority of Forerunners - during the Forerunner-Flood war in ancient past, prompting the activation of the galaxy-sterilizing Halo Array in 97,445 BCE.
Originating as a corrupted form of the universe's most ancient custodians (the Precursors) the Flood reproduces and spreads by infecting other organisms, hijacking their bodies and nervous systems in order to transform them into one of many specialized forms. Through infecting sentient lifeforms, the Flood assimilates their memories and intelligence, and the species becomes collectively more intelligent as a result. Because of their frighteningly rapid growth rate, as long as enough hosts of sufficient biomass and intelligence are available, the Flood is effectively unstoppable.
All Flood forms share a single consciousness, which becomes progressively more intelligent and sophisticated with every sentient host that is assimilated. When enough hosts have been infected, the Flood consciousness becomes a Gravemind - a self-aware, highly intelligent entity that strategically commands all Flood forms in an outbreak. Due to this group consciousness, the Flood as a whole could be more accurately described as a single, transsentient macro-organism rather than a collection of individuals directed by one or more controlling beings. In the absence of a Gravemind, the Flood possesses only basic coordination; they are incapable of forming complex strategies, and focus solely on attacking and infecting nearby sentient organisms. This stage of a Flood outbreak is known as the Feral Stage. At this stage, the Flood seek to create an assembly of biomass, calcium, and nervous system reserves, which leads to the creation of a Flood hive and the beginnings of a proto-Gravemind. The nature of the Flood's collective consciousness could be likened to a socialist utopia, due to the fact that the Flood act as a unified entity, with no individuality that would be present in other, more traditional species; each component of the Flood meta-organism works tirelessly for the advancement of their species.
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 the greatest threat to the existence of life (or, more accurately, biodiversity) in the known universe.