You prolly missed my question so I'll just say it again
Where did the flood come from?
Now this is a long and interesting tale.
10 million years ago, the Precursors created the forerunners and humanity. At first, they judged humanity to be those who would uphold the Mantle of Responsibility, something the Forerunners didn't like- so they rose up and exterminated the precursors in fear, then took the mantle upon themselves. Those precursors who did not get killed fled and reduced themselves to some kind of dust/ powder.
This powder was eventually found by Humans on a ship drifting in space; they began using it in experiments and to alter behavious of certain animals. After several centuries, the animals started to mutate and began the first outbreak. T first humans tried to wage war against the Flood themselves, wiping out planets to stop the threat, believing the Forerunners would take too long to do anything about the threat. The Forerunners saw humans wiping out Forerunner planets and waged war, eventually reducing humans to a Tier 7 species (cavemen) and planting them on Earth. Shortly after this, the Flood began fighting te Forerunners, still recovering from one war, and crushed them.