The Flood: a species so eminently threatening that a powerful and ancient civilization constructed habitable rings ten thousand kilometers 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 pseudo-parasitic, and have a simple procedure for reproduction.
There are three known types of Flood. There are the Spore class of Flood, the Carrier class of Flood, and what seems to be a Worker/Soldier class. To explain how the process begins would lead into a chicken-and-the-egg discussion, so we'll begin at the Spores. The spores are small, tentacled creatures which have their own defined biological framework, which isn't dependent upon a host. A spore will seek out any life of capable biomass to sustain itself, and proceed to attempt using the creature as a host. From here, it is assumed that one of two changes will occur in the host.
The first is that the host will remain physically unchanged, save for the spore 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 (it IS possible that Carriers are simply fully-grown Spores). The Carriers grow large epidermic sacs which contain several Spore-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 Spores in order to infect more hosts, and perpetuate the cycle of reproduction.
But 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. They are able to fire weaponry and repair damaged electrical and mechanical machinery at a surprisingly nominal level. It would seem that the Flood have some ability to access host memory, and utilize it to their advantage.
