The Kharaa is a highly infectious bacteria that was brought to Planet 4546B by the Architects, which forced them to put the planet under quarantine.
A strain of bacteria called Kharaa originated on an unknown planet. It was later discovered by the Precursors who unknowingly spread it to other planets, one of them being 4546B. Once they realized how much of the planet's inhabitants were infected, they put the planet under quarantine to prevent the Kharaa from spreading 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 Kharaa 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 Kharaa. Later the Precursors discovered a massive creature on 4546B that had a bizarre immunity to the Kharaa called the Sea Emperor Leviathan. They captured the creature and experimented on it and it's young in an attempt to find a cure to the infectious virus. As time went on, the Precursors later disappeared, either because they evacuated the planet or died off by the virus, and the Sea Emperor Leviathan was left in an experimentation 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 surviving human (the character you play as) later became infected by the Kharaa 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 Kharaa called Enzyme 42 and would spread the enzyme when set free to the oceans of 4546B, ultimately killing the Kharaa and curing all lifeforms infected (including the human). The SEL ultimately died fulfilling its charge of keeping its species alive and eradicating the Kharaa. The human later disabled the Precursor gun and escaped the planet.
After two weeks of incubation in whatever organism it infects, the Kharaa 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 bio-luminescent cysts on the skin. The body of the host would eventually shut down due to its inability to function after the symptoms.