Kharaa Bacterium

The Carar is a type of bacterium that seems to have completely taken over Planet 4546B. It is described in the Degasi PDAs as "part of the ecosystem". It is suggested that the Degasi survivors were infected at some point, as stated by Bart Torgal. It is stated in Precursor data that the Carar had killed 143 Billion creatures on the planet. The virus was released after a Sea Dragon Leviathan escaped and destroyed the Secondary Bio-Research Facility, resulting in the base's destruction and release of the Carar onto the planet. 

Afflicted creatures will be identifiable by the small, glowing cysts on their body.

Warpers will attempt to kill infected fauna once their infection passes a certain level.

The player is infected with the Carar, though it only becomes noticeable when the player reaches the Lost River Precursor Base. There is a scanner in the Precursor Ground-To-Air Weapon System that prevents access to the control room if the life form is infected.

If an infected creature is placed inside an Aquarium or Alien Containment, the creature will infect the rest of the creatures inside soon after. However, this is purely cosmetic and has no impact on the gameplay.

It seems that the only way to cure Carar is with enzymes produced by the Sea Emperor and its young.

The Sea Emperor and the Warper are most likely immune to the Carar, as they both play a role in eliminating it, respectively. 

Trivia

• The way that Carar manifests itself is loosely based off the appearances of the Kharaa from Natural Selection 2, as the Kharaa have similar looking cysts on their body.