I think perhaps you should read through things differently. For one thing, the Precursors didn't sit back all nice and idle while the Forerunners hunted them. They fled far and wide, and the forerunners followed. Some likely chose to fight, others ran, and then some hid. As for ancient humanity supposedly finding a cure for the flood, I think it ties in wonderfully to how manipulative Graveminds are shown to be. In past halos, Bungie halos I might add, here are some examples. Before the Forerunner saga was released, it was already known that a gravemind had fooled the forerunners most powerful AI, Mendicant Bias, into turning on them. During Halo 2, the gravemind sent John to High Charity to stop the Prophets and Thel to Delta Halo to stop Tartarus. It then turned on them once they either completed or failed in their goals. In Halo 3, the gravemind taunts Chief with messages of Cortana's decay and pain that it inflicted upon her. Again, in Halo 3, it temporarily allies itself with Chief and the Arbiter to to stop Truth, and then turns on them. So even before these novels were written, the Flood and their puppeteers already have a history of being manipulative, using ploys. SO why would it be so hard to believe that they too used a ploy on ancient humanity and the Forerunners? Humanity sacrificed great numbers in the belief that their cure worked, which in turn crippled a potential ally of the Forerunners. And when the Forerunners were up on the chopping block, they devoted valuable time and resources to finding a cure that wasn't there, instead of focusing on the threat. And then we come to the whole Flood is precursors thing. As I said above, most were killed, the few that survived were the ones that hid. And even they didn't live. Their powder became defective, and through it, the Flood was born. The remainder that survived saw this unexpected creation, and saw harmony through it, a tool to end the conflict of organic life, and gave themselves to it, therefore ending the Precursors and giving rise to something else, The Flood. The Flood contains the knowledge of the Precursors, and every victim it has ever claimed, wrapped into one eternal consciousness. But they aren't necessarily the precursors themselves. They are what is left. And now, they are even less so, as it has been shown that the Flood evolve and change rapidly, and are capable of hyper adaptation no other life form could ever hope to experience or reach.

Son of a gun there's actually some discussion going on here. Now then gentlemen, may we discuss what I originally posted. I'd like to hear some words from other folks about this.