But if even a single flood infected ship survived it would have all been for not, especially seeing as the receded the galaxy right after. I will have to look into the burns. You are right in regards to the food in halo 1 but I believe they escaped containment and survived for hundred thousand years but I'll have to look for the quote. Also with Forerunner technology they simply could've slowed down time so that mere minutes to them could've been centuries to the people outside (as my early let quote shows they could manipulate time within the fractal dimensions) and just take over the Galaxy then.