The Terothka virus was a highly-contagious disease which affected Romulan physiology. While various strains existed, the symptoms were similar – the most unpleasant of which were considered to be stomach cramps and a debilitating rash.
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")