I was thinking about Halo's methods of destruction the other day and realised that there may be a floor in its underlying principle.
Firstly look at a few of Halo's cannon facts 1. G617g was the planet the flood where encountered, they lay 'DORMANT' there 2. Halo kills the floods food
So my question is why did the flood survive on the planet G617g? They had no 'food'