The Terothka virus was a highly-contagious viral disease which affected Romulan physiology. Various strains existed, one nasty one being characterized by unbearable stomach cramps and a rash even worse.

While on board the hijacked USS Prometheus in 2374, The Doctor used the Terothka virus as a ruse to get onto the bridge of the vessel and attempt to incapacitate the Romulan crew aboard. The ruse was discovered when The Doctor's tricorder was found not to be taking scans of the crew as he claimed. (VOY: "Message in a Bottle")

It is possible that The Doctor made up the virus in order to proceed with his mission, as this is the only time to date it has been mentioned.