Trichomoniasis, sometimes referre to as "trich", is a common sexually transmitted disease (STD). It is caused by the single-celled protozoan parasite Trichomonas vaginalis. Trichomoniasis is primarily an infection of the genitourinary tract; the most common site of infection is the urethra and the vagina in women. It is most common in women and uncircumcised men. For uncircumcised men, the most common site for the infection is the tip of the penis.