Please, if you know as a fact, correct me if I'm wrong here.
The flood can NEVER die.
To my understanding, even the Halo array can't totally kill the flood. What it WILL do is kill everything of sufficient biomass for the flood to take over. The flood is like a virus. It doesn't die of starvation. What does die is its host bodies. The forerunners found out that they couldn't kill the flood using conventional means, so they decided to take away all available hosts. This stops the flood, killing all of the infected things and keeping the flood from expanding. But there are still flood there, but its not really alive, but at the same time its not dead. The flood spores left over have the potential to bring devastation, but no means by which to do it.
One thing that I want to clear up that I know as a fact. The ark wasn't built so the forerunners could take out the flood and come back. It was there to protect the Mantle. The Mantle, put very roughly, is the forerunner's mission to keep everything from going extinct. The Librarian went around and collected every living thing in the galaxy and put it on the ark so that once the rings were activated, they could come out and repopulate the galaxy (Biblical reference anyone?).
When life gives me lemons,
I make beef stew.
Gt: Mr Sticky100
New FF: The Final War
I would appreciate comments/critisism