Human-Flood war

The Human-Flood War was a war fought between ancient humans and the nascent Flood. It took place concurrently with the Human-Forerunner War, and was one of its primary causes. The war destroyed several human-populated systems and cost more than a third of humanity's population. Also, it destroyed hundreds of worlds and was one of the reasons that forced humanity into war with the Forerunners and also contributed to their defeat.

Contact
Early human ships discovered an unknown cargo ship near the edge of the galaxy. The humans found no crew or technology, but they found hundreds of small glass cylinders filled with an unknown powder. While tests found the powder to be harmless and useless, they began to test it out on smaller domesticated animals, one of which being the Pheru. The powder tests on the Pheru proved to be initially harmless, and they released the infected Pheru into the main population. Over time, the infected Pheru began to infect others and even began showing some symptoms of the Flood mutation, such as tentacles. Close contact with the Pheru spread the infection to humanity. Some San 'Shyuum were also infected as the Pheru were also kept as pets on the San 'Shyuum worlds. Eventually the Flood began to spread, and the Feral Phase was initiated.

War
By the time the human officials discovered what the powder had done, it was too late. Entire planets were infected, and the Flood began to ravage worlds and systems. This also initiated the Human-Forerunner War, because humans, blinded by greed and desperation, began forcefully taking worlds from other species to make up for the ones they lost to the Flood. When the humans attempted to quarantine the worlds, more infected escaped, and thus a drastic measure was invented. Despite how sacrificial it was, it worked.

The human leaders decided to take a third of the population and genetically alter them by inserting genes designed to fight Flood biomass and destroy the Flood gene imprints. They then took this third of the population and "fed" them to the Flood. The new genes destroyed the Flood until the few that survived were forced to escape. They would not reappear for another 9,000 years.

Aftermath
The Human-Flood War ravaged humanity and forced them into a war. Humanity began irrationally conquering new worlds and species, taking new planets to replace their own. This caused the Forerunners to act according to their Mantle, and declared war on Humanity. This also weakened Humanity's war front, as they were forced to fight the Flood and the Forerunners at the same time. With the Flood ravaging human worlds on one side and the Forerunners pressing closer on the other, Humanity was doomed. Eventually when the Forerunners won, they devolved humans, making humans a Tier 4 species.

List of appearances

 * Halo: Cryptum