Kharaa

The Carar is a highly infectious virus that spread throughout Planet 4546B and forced the native Precursors to put the planet under quarantine.

History
On Planet 4546B, a stain of bacteria called Carar was began to spread throughout the planet. It was later discovered by the Precursors of 4546B and once they realized how much of the planet's inhabitants were infected, they put the planet under quarantine to prevent the Carar to spread outside the planet. To do this, they created a gigantic structure that acted as a giant ray gun that would shoot down any spacecraft that breached the planets atmosphere. They also genetically created a race of aquatic fauna that was immune to the Carar virus called Warpers (because of their ability to teleport) which would be set free into the oceans of 4546B and hunt down and kill any living creatures that were infected by the Carar. Later the Precursors discovered a massive creature on 4546B that had a bizarre immunity to the Carar called the Sea Emperor Leviathan. They captured the creature and expirimented on it and it's young in an attepmt to find a cure to the infectious virus. As time went on, the Precursors later disapeared, either because they evacuated the planet or died off by the virus, and the Sea Emperpr Laviathan was left in an expirimentation facility for an unknown amount of time. Later, a human space exploration ship called the Aurora was shot down by the Precursor ray gun and crashed on the planet with all but one human being killed in the ship's destruction. The single human (the character you play as) left alive later became infected by the Carar virus while exploring the planet and discovering the various lifeforms inhabiting the planet and structures created by the Precursors. After a human rescue ship was destroyed trying to land on 4546B, the human tasked itself with shutting down the Precursor gun, which was set to not disable if the sapient being shutting down the gun was infected. Later, the human discovered the disease research facility the Sea Emperor Leviathan, whom the human learned was immune to the virus, and set it's young free by following the instructions of the Sea Emperor Leviathan telepathically. The baby SELs (Sea Emperor Leviathans) would produce the cure to the Carar called Enzyme 42 and would spread the enzyme when set free to the oceans of 4546B, ultimately killing the Carar and curing all lifeforms infected (including the human). The SEL ultimately died fulfilling it's change of keeping it's species alive and killing off the Carar. The human later disabled the Precursor gun and escaped the planet.

Infection Symptoms
After two weeks of incubation in what ever organism it infects, the Carar spend another five weeks attacking the body by shutting down the immune systems of their hosts, which makes the host more susceptible to other diseases, and genetically mutate their DNA with the creation of various bioluninescent cysts on the skin. The body of the host would eventually shut down due to it's inability to function after the symptoms.