trichomoniasis, sexually transmitted disease caused by the parasitic protozoan Trichomonas vaginalis. In women, it can cause urinary tract infection and a painful, malodorous vaginitis marked by a thin, foamy, irritating discharge. In men, it can infect the urethra and bladder. Most men have no noticeable symptoms, an important factor in its easy transmissibility. Trichomoniasis has been linked to the birth of low birth weight or premature infants and may increase the risk of AIDS virus transmission. The infection is treated with metronidazole (Flagyl) or tinidazole (Tindamax, Fasigyn).