Trichomoniasis is caused by the protozoan Trichomonas vaginalis. It damages the epithelium of vaginal and urethral tissue and more than doubles the risk of acquiring human immunodeficiency virus (HIV). This infection has an incubation period of 4 to 28 days and is associated with other STIs, such as gonorrhea, chlamydia, and HIV.

It was once thought that trichomoniasis resulted from common water in hot tubs, wet bathing suits, and wet towels because trichomonads can survive for 45 minutes outside the body; however, this is now thought to be unlikely. It is transmitted by penis-to-vagina intercourse or vulva-to-vulva contact with an infected person. Trichomoniasis is not transmitted by oral or anal sex.

Risk factors for contracting trichomoniasis include the following:

Complications of trichomoniasis include prostatitis, urethral stricture disease, and epididymitis in men. In women, pelvic inflammatory disease may occur. Trichomoniasis may also play a role in cervical neoplasia and postoperative infection as well as in acquiring herpes simplex type 2. Infertility can be a complication for both men and women. For pregnant women, preterm birth or adverse fetal outcomes may result from untreated trichomoniasis.

Symptoms are different in men and women. Some patients, especially men, are asymptomatic. When symptoms do occur, women may have a history of dyspareunia and dysuria, along with a frothy, foul-smelling, yellow-green vaginal discharge. A patient may complain of vaginal soreness and itching, and there may be vulvar or vaginal redness. If cervicitis is present, purulent discharge and bleeding may occur. The patient may also complain of abdominal pain or tenderness. On gynecologic inspection, the cervix may show a patchy macular erythematous lesion. Men may complain of urethral discharge and itching as well as dysuria. (See Symptoms in trichomoniasis.)