The Omega IV virus was from the planet Omega IV. Possibly initiated as a biological weapon, the virus will turn most Humanoid life into a pile of crystals. The 'antidote' for the process is to remain on Omega IV until the immune system can adapt to the virus.

In 2268, the virus killed all the members of the USS Exeter's crew except Ronald Tracey. (TOS episode: "The Omega Glory")

The Albino used variations on the virus to control and kill. (Excelsior: Forged in Fire)