- last post: 01.01.0001 12:00 AM PDT
I've I had to guess, I'd say that the flood evolved naturally on a planet. The planet had creatures which the flood infected, but they had no way of getting off. The flood, from reading the Halo books and from evidence in the game, are able to sort of sift through memories of the host creature. If the other creatures of the planet are not sentient or have not reached space travel yet, the flood is trapped on the single planet.
Eventually the forerunners came across the planet. When they got attacked by the flood, starships were probably captured, as we've seen happen in the halo series. The flood can now spread rampant around the galaxy. It's like if all the bugs on earth attacked humans. Sure we could kill tons of 'em, but there's so many and would reproduce so quickly that it would be immpossible to survive.
The forerunner then built the halos in as a last resort. If they don't activate, the flood consumes them all. If they do, at least life in other galaxies will survive and life here can start over.
The forerunner would no doubt also be trying to find a better way to eliminate the flood. After all, they don't really want to wipe themselves out as well. So wouldn't it make sense to take infection forms and study them? If I recall correctly, humans on halo took dead, but mostly intact, ones to study. (The Flood) Also, Dr. Halsey had composed a report on the flood for ONI to try and come up with a prevention/cure for the flood. (First Strike)
The forerunner no doubt preserved flood in a controlled enviroment to study live samples. These flood would survive the activation of the Halos, because the halos themselves were uneffected. You'd think that the halos would destroy themselves by creating a blast of that magnitude in their center, but they must have had some sort of shielding, incase they were needed to be activated in the future. This shielding also kept the flood on the ring alive.
They of course had to feed them to study them live. I would assume that they had automatic feeding systems in place since walking in there to get infected would be stupid. But what happens when a bunch of religious crazed elites barge into the containment area looking for the path to their great journey?