As said, Precursors were the only race who have achieved the level TIER 0 , they were exceedingly intelligent. They created the Forerunners from a very primitive race in the galaxy , only one existed naturally in their time , and accelerated the evolution of intelligent life of forerunners.
As forerunners became intelligent as nearly as the precursors , they started conflicting with Precursors for power and superiority. As precursors were a highly intelligent and noble species , they'd by-the-time had disarmed their weapons which caused their defeat in the hands of forerunners.
But a handful of precursors , who escaped the forerunner wraith ,in the on-going war , by traversing out of the galaxy , deliberatly deploying the species , known as "flood" by forerunners and other species in the galaxy , as their last option for defense and offence against the forerunners. But the date-time anomaly for as the first contact with flood with forerunners is unknown because forerunner-flood war started after millions of years after the forerunner-precursor war.
NOTE : flood were a species which the precursors were creating by-themselves , writing its DNA structure on their own, to create a lab-fabricated species which eventually will dwell the galaxy as other naturally spawned species.But when they were creating this species , precursor-forerunner war started , all their attention from this project shifted to the war.As the war enraged , much less time and attention was given to this project and was completed in a hurry without any testing.It is in question that the project , in a hurry "went horribly wrong" or was made to go "horribly wrong " to use it against the forerunners.
um, where the hell did you learn all this? i never read ANYTHING remotely close to the precursors made the flood, and that the forerunners warred with the precursors, who, if im not mistaken, were either extinct by the time the forerunners evolved that far, or had moved on to another galaxy, and that the flood were an extragalactic species, im not denying that the precursors helped the forerunners evolve, as the forerunners had the power to reset and then help humanity as a species evolve(prior to the flood forerunner war, see the forerunner novel for more info), as humanity was also equally evolved to the point of having a cure for the flood but losing nearly all human worlds before gaining said cure and preparing to use it, attacking forerunner worlds to have places to live, gaining the anger of the previous owners of said worlds, beinf de-evolved to caveman, and destroying the cure out of spite so the forerunners would be unprepared for the coming flood threat, which they were(the novel clearly staes forerunner and humanity were both tier 1 before the flood attacks, humanity was devolved to tier 7 and the forerunners were forced to use halo, while humanity's dna was on the ark, saving the race, the forerunners werent, and they programed all their tech to respond to the only other race worthy of using it, humanity)