SPOILER! I read all 3 Forerunner Saga books. In Cryptum a young Forerunner named Splendid Dust of Ancient Suns explains the background of the Flood to Bornstellar Makes Eternal Lasting. Splendid tells that the Precursors changed themselves into dust, that dust would change them back to their biological form when the time was right. In Silentium Catalog discovers that the Forerunners drove the Precursors from this Galaxy, some of them fled and some of them turned into dust to regenerate overtime into their biological form when the time was right. But they waited too long and the dust became nothing else than sickness and disease. And so their new form wasn't Precursor but Flood. So that means that the Precursors [u]didn't create[/u] the Flood as a test for the Forerunners and the Humans to see who the real heir is to the Mantle, but they [u]became [/u]the Flood. So that's why Humans didn't became infected anymore by the Flood. And that why the Forerunners thought about a cure that never existed. And so the Precursors always were a step ahead of the Forerunners. As their personal revenge, the Precursors planned this milions of years before. And so the Humans became the rightful heirs of the Mantle, the Reclaimers. But then i ask why. Why does the Flood/Precursors attack Humans again during the Covenant war? Didn't they want the Humans to become the keepers of the Mantle?