The Flood

The Flood are parasitic alien lifeforms that are driven to infect any to all sentient life they encounter, and were a grave threat eons ago.

The threat was so bad, that it forced the Forerunners to kill themselves and other sentient life in order to starve the Flood to death.

Before doing so, the Librarian re-seeded all sentient life and/or relocated them to the Ark.