From Halopedia, the Halo wiki
- Circa 107,445 BCE, human colonists proceed to uncover crashed vessels containing fine organic powder on both inhabited and deserted worlds near the edges of the Milky Way Galaxy. This powder is administered to the domesticated Pheru, which begin to exhibit more docile behavior; unbeknownst to humanity and their San'Shyuum allies, the powder begins to alter the Pheru's genetic makeup. Centuries later, the Pheru begin to show outward signs of mutation and cannibalize one another. Soon, both humans and San'Shyuum who are exposed to the powder begin to exhibit the same symptoms. The Flood then begins to take over hundreds of worlds, turning its victims into misshapen forms. Humanity begins all-out war against the Flood. During their war against the Flood, humanity desperately invades a minor sector of Forerunner space, sparking the human-Forerunner wars.