Kharaa

Kharaa is an alien bacterium discovered by the Precursor Race during the exploration of an unknown planet. It seems to have completely taken over Planet 4546B. It is described in the Degasi PDAs as "part of the ecosystem", with further suggestions that the Degasi survivors were infected at some point - as stated by Bart Torgal while discussing the flu-like symptoms the crew was experiencing. Early on, the player's PDA and Lifepod will make note of the high levels of bacterial infection in the water.

It is stated in Precursor data that the Kharaa bacterium has killed over 143 billion individuals over the course of time since its discovery. It is possible this is not the true death count as the PDA entry only lists these as "Confirmed Deaths" which could imply further dead beyond what was known. Their core worlds were quarantined off, and research facilities such as the one on 4546B were created to find a cure. After a Sea Dragon Leviathan damaged the Disease Research Facility in an attempt to retrieve its offspring, the previously contained Kharaa bacterium was released onto the planet. This decimated the planet's ecosystems, as much of the life on the planet died due to the bacterium's immuno-repressive behaviour and genetic corruption. This prompted the Precursors to initiate their emergency quarantine: activating the Quarantine Enforcement Platform. The Quarantine Enforcement Platform shoots down all approaching or departing spacecraft to prevent Kharaa being spread to other planets.