A sexually transmitted disease (abbreviated STD) is a disease that can be transmitted by sexual contact with an infected individual.
DoctorVonus explained that a common symptom of a sexually transmitted disease, in a joined Trill at least, was the acute onset of pain. An elevated white cell count also could be caused by a sexually transmitted disease. Vonus also stated Starfleet doctors frequently ignored sexually transmitted diseases as a possible diagnosis in cases. (Henglaar, M.D.: "Risk Factors")