So yeah, who created the flood... You know the parasite witch apears at halo 1;2;3.
cybertron thinks humans created flood before forerunner-human war, he thinks that is was a unsucsesfull experiment, and broke out.
I think that precursors created flood during forerunner-precursor war, as a failsafe incase that forerunners defeat them.
Or my 2nd theory is that none of them created it and the flood was created in distant galaxy, or was dormat on G 617 g