E Parker writes:
My theory on the Flood are that they are a multi-celled virus, and we, are their multi-celled host "cells." As the Flood only take the live ones, so too do our one-celled not-so-friendly DNA-in-a protein covering, aka the virus.
Also, like many invertebrates inhabiting caves and deep oceans, perhaps the Flood enjoy the ability to come together to form a "community," all working toward the benefit of the whole. Certain flood may be components of an arm or an eye, a leg or a claw. In order to unite, however, they'd need to do their own version of terra-forming in which they alter the atmosphere. This is what GS was talking about when he complemented the Chief on being "a good planner."
This reminds me of another aspect of ::gasp:: the Vang (by Christopher Rowley). The Military Form has a strong genetic compulsion to produce a Higher form, a ruling Hostmaster, but only when the surroundings and foreseeable future seems reasonably secure enough to do so safely. Is atmospheric change merely a precursor to something more?