Trichomoniasis is a sexually transmitted disease (STD) caused by a small organism called Trichomonas vaginalis. Women are most often affected by this disease, although men can become infected and pass the infection to their partners through sexual contact. Trichomoniasis is the most common curable STD in young, sexually active women. An estimated 7.4 million new cases occur each year in women and men. Men often do not have symptoms of trichomoniasis and usually do not know they are infected until their partners need treatment.