Trichomoniasis Overview. Trichomoniasis is a sexually transmitted infection (or sexually-transmitted disease, STD) caused by a protozoan (a microscopic parasite), usually found in the vagina and urethral tissues.Although this condition is most often treated in women, trichomoniasis infection in men can also occur (and often has no symptoms).richomoniasis Overview. Trichomoniasis is a sexually transmitted infection (or sexually-transmitted disease, STD) caused by a protozoan (a microscopic parasite), usually found in the vagina and urethral tissues.