The Flood, designated as LF.Xx.3273 by the Forerunners and referred to as the Parasite and the infection by the Covenant, 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 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 galaxy'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 is more accurately described as a single, transsentient superorganism 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 Milky Way galaxy.
The Flood is composed of a variety of specialized forms, each filling a different role in combat. The most basic Flood forms are the infection forms, which come in various types and infect hosts by hijacking their bodies and nervous systems. Infected hosts then undergo a rapid series of transformations, becoming lethal combat forms. Combat forms are used to subdue and secure additional hosts for infection, and once they have outlived their usefulness as warriors, they transform into carrier forms, spreading the Flood by spreading more infection forms.
Once the Flood has amassed enough bodies to create a Gravemind, it is capable of generating "pure" forms, which are created solely from assimilated nutrients and biomass. The pure forms observed during the outbreak in late 2552 were able to spontaneously mutate between three configurations: Stalker, Tank and Ranged forms, although in practice the true extent of their mutability is impossible to effectively catalog.
The Flood at this point relegate the combat forms to defensive purposes or as additions to the biomass, calcium, and nervous system reserves of the Flood hive, and rely increasingly on the pure forms as their primary instrument of infantry force. However, in order for the pure forms to fully replace the combat forms, the pure Flood population must be large enough and in a state of equilibrium, which is in a constant state of flux depending on Flood gains and losses. Because of this, it is common for the combat forms to serve alongside the pure forms, in addition to the infection and carrier forms, as the acquisition of hosts and technology would likely outstrip the speed in which the Flood can generate pure forms.
Pod infectors: These are among the most basic of Flood forms. They are tasked with seeking hosts, then infect them that mainly on the ground.
Tank form: These large tanking forms can only be produced when a Flood outbreak has amassed considerable amounts of biomass and formed a Gravemind. These forms can take and deal massive amounts of damage.
Ranged form: These flood forms can only be produced once a Flood outbreak has obtained enough biomass to form a Gravemind. These forms attack via bony spikes shot from it's back area, however is immobile.
Stalker form: These pure forms can only be produced if a Flood outbreak has gathered a large quantity of biomass to create a Gravemind. These forms are very fast and agile.
Carrier form: These bloated forms incubate Pod infectors, and release them upon death.
Combat form: Once a host of any organic/sentient species has been infected, they can develop into a combat form taking the appearance of a mutated and infected version of the infected species.
Gravemind: This ultimate Flood form serves as the coordinating hive mind of the Flood. While they vary in size and complexity, at their height Graveminds can engulf entire worlds.
A Flood infestation goes through four distinct developmental stages:
During the Feral stage, the Flood are at their simplest form: they communicate via pheromones and have the instinct to harvest enough calcium to establish a viable Gravemind.
The Flood become truly dangerous during the Coordinated stage: at this point, they are controlled by the Gravemind that was created in the first stage.
In the Interstellar stage, the Flood take control of space-faring technology which they use to consume the local star system. Subsequently, they spread throughout the galaxy to infect more hosts.
The Transgalactic stage is a theoretical stage in which the Flood utilizes all captured technology to depart to uninfected galaxies to further replicate. At this stage, the Graveminds collective consciousnesses merge to form a "Super Gravemind" capable of controlling all of the flood throughout a single galaxy.
There is also a theoretical fifth developmental stage in which the flood is able to infect multiple galaxies resulting in the consciousness of multiple "Super Graveminds" to merge. It is unknown what the result of this merge would be but the Forerunners believed it would result in the creation of a being that would be able to destroy all of reality. The fear of this outcome is what pushed the Forerunners to quickly complete the Halo array and fire it in order to stop the Flood infection as quickly as possible.