The Flood is opposed the start of Halo along with the Forerunner, as the Flood landed on the Forerunners planet as an invasion they would come and take control of other life forms feeding on them as it seems to look like they are zombies in some sort of way. As they can take control of Forerunners they were able to get ahold of their technology causing them to expand to other planets. This had caused the Forerunner to build the Halo Rings, and copied every known living things DNA. After this was done the Forerunner activated the rings annihilating every life force in the universe, while preserving the Flood contained and bringing every life force back; this granting life. As that was supposed to be the end of the Flood, As then brings the journey of the rings and the release of the Flood.