A species so eminently threatening that a powerful and ancient civilization constructed habitable rings ten thousand kilometres
in diameter to contain and observe them. The Flood are as numerous as they are voracious; they neither surrender nor retreat.
The Flood appear to be sentient and omni-parasitic, preying upon any host utilizing a certain minimum level of sentience and
bio-mass, and can infiltrate "even advanced life forms". It is not known whether they were merely encountered, or
created by the Forerunner for some purpose known only, as of yet, to themselves.
There are five known types of Flood. There is the Ranger/Infection class, the Carrier/Incubator class, a Worker/Soldier
class, a Command/Pilot form (as contained Keyes), and the Gravemind intelligence (it is not known if the Gravemind is a solitary
individual or one of many such beings). To explain how the process begins would lead into a chicken-and-the-egg discussion,
so we'll begin at the Rangers.
The Rangers are small, tentacled creatures which have their own defined biological framework, which are not dependent
upon a host. A Ranger will seek out any life of capable bio-mass and calcium deposits to sustain itself, and proceed to attempt
to use the creature as a host, by tapping into the spinal system, suppressing the host's consciousness, embedding itself in
the thoracic cavity, and releasing spores which cause the host to mutate. (343 Guilty Spark mentions these "spores"
during his own synopsis of the Flood life-cycle, which may be either another term for the Infection form, or an even more
base form of the Flood.) From here, it is assumed that one of three changes will occur in the host.
The first is that the host will remain relatively physically unchanged, save for the Ranger attached to it. These would
be the Worker/Soldier class, used for manual labor such as building, gathering, repairing, or defensive/offensive actions.
So far, only Marines and Elites have been seen to mutate into Workers/Soldiers.
The second host possibility is that of the Carrier, which presumably only affects creatures who are physically or mentally
unsuitable to be Workers/Soldiers. The Carriers grow large epidermic sacs which contain several Ranger-class Flood. When a
Carrier is in close proximity to any number of suitable hosts, it triggers some manner of biochemical reaction, causing the
pus-filled sacs to explode and spread the Rangers in order to infect more hosts, and perpetuate the cycle of reproduction.
When the Flood's inability to suitably care for the host body has caused sufficient degeneration, the Worker/Soldier class
will also change into the Carrier form. (It is not known whether the "Juggernaut" hidden on the H2 disc is a Worker/Soldier
of another race, or yet another form entirely.)
The Command form has been seen in detail only once and discussed even less, therefore the process of its creation remains
uncertain. Found on the bridge of the Truth and Reconciliation, this large multi-appendaged amalgamation appears to be a combined
symbioses including several, at least in this case, Human hosts. Other than extracting information, its capabilities and roles
are unknown. On High Charity, other such growths are seen lining the walls and apparently even piloting Pelican dropships,
though this time with no obvious hosts.
The nature of Gravemind taunts us all the more...
Though they have a relatively simple procedure for reproduction, as 343 Guilty Spark states, the "parasitic nature
of the Flood belies their intelligence". What is interesting is that they are able to function the machinery and technology
of their host to an amazing degree, and are able to fire weaponry and repair damaged electrical and mechanical equipment at
a surprisingly nominal level. The Command form (as we learn through Keyes) has the ability to access host memory, strip it
from them piece by piece, and utilize it to it's advantage; the lesser Flood also possess some semblance of this ability,
though from what we have seen (Jenkins) it does not appear to have such a pronounced sense of self; it does not exert the
same levels of mental force or operate nearly as methodically.
And although the Halo network is meant to ultimately starve and eradicate the Flood, their continued existence after a
previous firing and eons of isolation shows ever more that though they may be contained and slowed, as a whole they are not
so easily destroyed...
Gravemind is, as of yet, a mystery. Cognizant, emotional, and cunning, he is a guiding force behind the Flood found on
Delta Halo 05. However, it is not known if he should be referred to as 'the' or 'a' Flood Intelligence, or whether he is a
singular source/leader or a Flood form which will eventually grow under the proper conditions. It is also unknown how he manages
to communicate with the other Flood and if he possesses motives and aspirations beyond the mindless hunger the Flood is typically
known for. He does appear to be capable of some rather impressive feats: utilizing the Halo's teleportation grid, assimilating
and linking with other life forms and machinery (Regret and 2401), and, in capturing John and the Arbiter, a sort of omnipresence.
With the annexation of High Charity, its tremendous biological and technological assets, and the willingness of Cortana, the
next move is certainly his...