Trichomonas vaginalis. Abstract illustration of the protozoa Trichomonas vaginalis (purple), the cause of sexually transmitted vaginitis, shown in magnified view inside a 0+" female symbol. The thread-like flagella on these protozoa enable them to move by whipping their fluid surroundings. The round pink cells with lobed nuclei are white blood cells, produced by the body to attack invading bacteria. Vaginitis is inflammation of the vagina. It is most often caused by sexually transmitted T. vaginalis, which causes irritation and an offens- ive, frothy discharge. The condition usually clears up following treatment with metronidazole."