Precursor, a race more advanced than the Forerunners, created the flood for revenage against the Forerunners
humans and forerunners are completely different races, which is why there was a forerunner-human war. Humans use to be just as advanced as forerunners, if anything a little more, but lost(fighting both forerunner+flood at same time, wasnt a fair fight) and so where de-evolved as punishment.
There has been at least 2 graveminds known, the other being in the forerunner-flood war. They are made from thousands of other minds and bodies that are absorbed together, and continue to absorb, to create one being, which is why they are so smart.
My guess for the start of the halo 4 storyline (gonna be 2 more games that follow) is about more to do with the forerunner history, artifacts and stuff. Since if i remember correctly he was apporching a use to be inhabited forerunner planet. There could also be forerunners still alive hiding in shield worlds(constructed places to surviving the halo array) which would be interesting. Flood could still easily make a return as an enemy.
This post was edited by nitrojsp on Nov 11 2011 02:15pm