AVERAGE SIZE: Variable by host and form
AVERAGE LIFESPAN: Unknown
COMMON NAME: Parasite
The Flood is one of the most unique species in the Halo universe. Its origins are still bathed in mystery, but it is known to have begun from the ancient Precursors, fleeing the wrath of the Forerunners at the end of the Forerunner-Precursor War. Millions of years after the Precursors were eradicated from the galaxy, ancient humanity discovered vessels abandoned at the edge of the galaxy, containing nothing of interest aside from a desiccated powder. The humans took the powder back to their home on Faun Hakkor, where it was eventually fed to their pets, the Pheru, and mutated them into dangerous, feral creatures. This mutation eventually managed to spread to the human population, and while efforts were made to eliminate the diseased humans, the mutated individuals ended up infecting enough others that containment was impossible. The Flood, or as it was known then the Shaping Sickness, was born.
The Flood spread among the human colony worlds, forcing ancient humanity into a retreat, and pushing them into Forerunner-controlled space which began the Human-Forerunner Wars. For part of this war, ancient humanity was forced to fight both the Forerunners and the Flood on two fronts, but at some point the Flood retreated out of the Milky Way galaxy, and while there was no known reason for this retreat, the Forerunners used it as evidence to show that humanity held a cure to the Flood parasite.
After humanity lost the war with the Forerunners, they were nearly wiped out entirely. It was only the supposed knowledge of a cure to the Flood that convinced Forerunner leadership to devolve humanity and keep the species alive rather than completely exterminate them. For thousands of years after, the Forerunners prepared for the inevitable return of the Flood via many means. Humanity was continued to be studied for a cure, and two separate construction projects were started: the defensive shield worlds championed by the Didact and the offensive Halo Array supported by the Master Builder.
Around 97,745 BCE the Flood returned to the galaxy with a vengeance, conquering countless Forerunner colony worlds very rapidly, and putting the Forerunners onto the defense. Although the less powerful Senescent Halo Array had been completed by this time, the Forerunner’s ability to use it against the Flood was almost immediately hampered by the Flood and the Primordial’s corruption of the array’s ancilla Mendicant Bias with the logic plague. Though it is unlikely that the Forerunner would have been able to stop the Flood even at full capacity, this corruption hastened the end of the Forerunner race considerably.
Although the Forerunners fought the Flood for around 300 years, they eventually were pushed back to the Greater Ark, where the last of the Forerunner military and known population made their last stand which the Flood quickly overrun and destroyed through the use of Precursor star roads. Only a very small few Forerunners managed to escape this battle, and the last remaining Forerunners fled to the Lesser Ark, which they used to take command of the galaxy-killing Neoteric Halo Array. The Flood made an attempt to stop them from firing the array, but the Forerunners managed to successfully light the rings and wipe out the Flood and all sapient life from the galaxy.
The galaxy’s species were reseeded and civilizations allowed to grow again for the next 100,000 years, until the Flood was released again during the Human-Covenant War. There were at least three different times the Flood was encountered during the war, and while the Gravemind managed to commandeer High Charity and nearly wipe out Earth in 2552, the threat was successfully contained and eliminated. Though the Flood has not been seen in a significant capacity since the end of the war, it is assumed the threat of the Flood remains somewhere within the Milky Way.