Parokki on 03 October 2007
Pretty much what everyone else is saying. The Flood had access to the same technology as the Forerunners, and running wouldn't have worked.
It's all a bit hazy, but I'm sure Halo 3 explains this at least a little bit. I've been playing with the idea that a bunch of the Forerunners places themselves in stasis and woke up waaaay later becoming the humans on Earth, but I HAVEN'T PLAYED THE GAME SO PLEASE DON'T TELL ME IF I'M RIGHT OR NOT.
