Trichomoniasis ("trich") is caused by a tiny parasite (a one-celled protozoan) that most commonly is sexually transmitted from an infected person.

In women, trich organisms usually infect the vagina, urethra, cervix, bladder, and glands in the genital area, such as Bartholin's and Skene's glands. See an illustration of the female reproductive system.

In men, infections develop in the urethra or under the foreskin of an uncircumcised penis. See an illustration of the male reproductive system.