Trichomoniasis, sometimes referred to as "trich," is a common STD 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 man, and the vagina is the most common site of infection in women.

Symptoms. Trichomoniasis, like many other STIs, often occurs without any symptoms. Men almost never have symptoms. When women have symptoms, they usually appear within four to 20 days of exposure. The symptoms in women include a heavy, yellow-green or gray vaginal discharge, discomfort during intercourse, vaginal odor, and painful urination. Irritation and itching of the female genital area, and on rare occasions, lower abdominal pain also can be present. The symptoms in men, if present, include a thin, whitish discharge from the penis and painful or difficult urination.

Treatment. Because men can transmit the disease to their sex partners even when symptoms are not present, it is preferable to treat both partners to eliminate the parasite. Metronidazole is the drug used to treat people with trichomoniasis. It usually is administered in a single dose. People taking this drug should not drink alcohol because mixing the two substances occasionally can cause severe nausea and vomiting.

Complications. Research has shown a link between trichomoniasis and two serious sequelae. Data suggest that trichomoniasis is associated with increased risk of transmission of HIV and may cause a woman to deliver a low-birth-weight or premature infant. Additional research is needed to fully explore these relationships.