The colony is actually an extermination campThrough out the movie everything the military did was forehead smackingly stupid. They left an infected women unguarded and unmonitored. Their approach to quarantine was to shove everybody together in a room with non-zombie-proof doors. They were quick to order "Kill Em All" and didn't even take simple measures to differentiate infected from clean (IE telling every one before hand "If the alarm goes off pick up a any object and start running, that way the soldier know who not to shoot"). Most damningly, the Army shot at a car from a helicopter, despite no zombie displaying any form of driving ability. There is simply too much stupidity on display for the colony to be accidental. Meaning that all that was done on purpose. My conclusion is the colony is the Final Solution to the British Problem. The plan was to gather all the British from all over the world and Concentrate them in one Camp. Then fake a zombie outbreak to exterminate them all in one fell swoop. Presumably the room they gathered in had Zyklon B dispensers, but the higher up saw a real zombies outbreak and thought "Meh! Why not?" and just didn't turn them on.
The rage virus has a non-leathal airborne aspect. It affects thinking and thus makes people make dumb, dumb decisions.This would explain why the researcher in the first movie didn't say the monkeys were infected with a bioweapon, he just said "They have rage!" It explains all the really dumb decisions we've seen people make in the movies. It makes sense from a survival standpoint - the virus needs people to act stupid to ensure its continued existence.
David Bowie is an asymptomatic carrier of the virusHeterochromatic eyes seem to be the sign of special blood.
Alice (and by extension Andy) are not completely asymptomatic carriers."Carriers" are spared the homicidal rage aspect of the virus, but still suffer from extreme emotional lability. This is why Alice was so hysterical in the ten minutes of screentime she had before being killed. Granted, she might have just been driven insane by living alone in a bedroom for seven months...