Okay so I may have read into this wrong, but from my understanding, in halo primordium, it turned out that the humans had infact not invented a cure to the flood, but the precursors just decided that the humans were the new inheritors of the mantel, so they had the flood stop infecting them.
Now, in halo 1-3, the flood does infect people. This has lead me to believe that the precursors for whatever reason had found the humans once again unworthy of inherriting the mantel. I believe that they may have created a third species, similar to both humans and forerunners, but much stronger, and more obedient; The Brutes.
It's far fetched I know, but I think it would make a good plot twist. Thoughts?