The Flood: 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 four known types of Flood. There are the Ranger/Infection class, the Carrier/Incubator class, a Worker/Soldier class, and what can be, as of yet, described only as a Command form. ( 343 Guilty Spark also mentions " 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 type of the Flood.) 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 biomass 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. From here, it is assumed that one of two 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 Covenant 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.
The Command form is seen 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.
Though they have a relatively simple procedure for reproduction, as 343 Guilty Spark states, the " parasitic nature of the Flood belies their intelligence". What´s interesting is that they´re 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 machinery 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...
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