The Omega IV virus was a disease, an infectious virus from the planet Omega IV. Possibly initiated as a biological weapon, the virus will turn most humanoid life into a pile of chemical crystals. The "antidote" for the process is to remain on Omega IV until the immune system can adapt to the virus.
In the year 2268, the virus killed all the members of the Federation starship USS Exeter's crew except Ronald Tracey. (TOS episode: "The Omega Glory")
The virus had similarly eradicated the crew of the ECS Philadelphia a century earlier. (DTI novel: Forgotten History)
The Albino used variations on the virus to control and kill. (ST novel: Excelsior: Forged in Fire)