Flood (Earth-2042)

"One single Flood spore can destroy a species."

- Rtas 'Vadum

The Flood, designated as LF.Xx.3273 by the Forerunners and referred to as the Parasite and the infection by the Covenant and the Galactic Empire, is a sentient 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 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 BBY.

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 is 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 is similar to that of a hive mind, the Flood act as a unified entity, with no individuality that is inherent to other species; each vessel of the Flood works tirelessly to aid in the propagation 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 widely considered to be the greatest threat to the existence of life (or, more accurately, biodiversity) in the galaxy.

Source

 * https://www.halopedia.org/Flood