The Flood are a parsitic life form creted by bungie for Halo: Combat Evolved. They also appear in Halo 2, Halo 3 and Halo Wars. The Flood are driven by the desire to infect any sentient lifeform they encounter and are such a threat that the Forerunnner's were forced to kill themselves 100,000 years ago in an effort to starve the Flood to death.
Origins== As the Forerunner-Precursor War drew to a close, the few remaining Precursors were pushed to the edge of the Galaxy to Path Kethona sought vengeance against the Forerunners. A few Precursors were spared by the Forerunners whilst others became a dust which could regenrate into their past forms. Over time this dust became so corrupted it created nothing but horrific diseases. The Precursors saw this as an oppurtunity to exact revnge on the Forerunners and released the Flood on them.
The Flood were an extremely adaptable, dangerous and expendable parasitic lifeform.
They were first encountered in the Milky Way Galaxy by an Ancient Human Empire 10,000 years prior to the Forerunner-Flood War. The earliest known instance of the highly infectiously viral super cell later known as the Flood form to be encountered, was a fine, desicated powder contained withign glass cylinders.
Early experiments showed potential for pssychotropic effects in some lower levels but not Humans or San'Shyuum. The primary animals affected were popular pets in human societies: the Pheru , livelt and gentle creatures found on Faun Hakkor.
very small quantities of the powder induced changes in the Pheru that improved their domestic behaviour.
The Flood seemed to enter the Galaxy from the Large Magellanic cloud on the edge of the Milky Way roughly 110,000 years prior to the Halo event, contained in cylinders in unmanned vessels where they were kept aas a powder. A considerable amount of time passed between the discovery of the powder by pre-historic humans and their evolution into their current forms. The Flood were eventually able to infect Human worlds, sparking both the Human-Flod War and as a consequence, the Human-Forerunner War. The Humans finally managed to create a cure for the Flood and eradicate it. It would be thousands of years before the flood would be able to threaten the galaxy again.
Unbeknownst to the Humans, they hadn't found a cure, but the Flood had chosen not to infect Humans as they had passed the test to attain the Mantle - in the process of stopping the Flood they had thrown a thrid of its population at the Flood.
"It fed on intelligent life and in doing so became ever more intelligent itself. I used their own strength aginst them"
Cortana talking to Master Chief about his origins
More than 100,000 years prior to the Halo event, the Flood encountered the Forerunner's on the planet G617 g1