Trichomoniasis, sometimes referred to as "trich" or the ping pong disease, is a common sexually transmitted disease that affects 2 to 3 million Americans yearly. It is caused by a single-celled protozoan parasite called Trichomonas vaginalis. Trichomoniasis is primarily an infection of the urogenital tract; the urethra is the most common site of infection in men, and the vagina is the most common site of infection in women.