Trichomoniasis, or "trich," is a sexually transmitted vaginitis suffered by an estimated 5 million American women yearly. In men, this infection rarely produces any symptoms; approximately 40 percent of infected women are also asymptomatic. Since they are unaware of their infection, these people unwittingly spread the infection throughout their communities.

Like most organisms responsible for causing sexually transmitted diseases (STDs), the trichomonad is fragile and cannot survive long outside the body of its host or hostess. However, infections have occasionally been traced to the use of shared washcloths or towels. Since the trichomonad dies promptly when exposed to drying, it cannot be caught from toilet seats, saunas, or dry linens.

Chemical purifiers in swimming pools and hot tubs will also kill it.

While infection with trichomoniasis can be intensely uncomfortable, it is not a serious threat to your overall health. Still, it does increase the risk of premature delivery in pregnant women and, as with other STDs, it can increase your chances of acquiring HIV. Antibiotic treatment cures it more than 90 percent of the time.

The most striking symptoms are vulvar and vaginal burning and itching. The burning may be most apparent after intercourse and can affect the skin of the penis as well as that of the vagina. In addition, there may be vulvar swelling and frequent and uncomfortable urination. There is a heavy vaginal discharge, usually yellowish or green, which may or may not have an offensive odor.

The trichomonad is a tiny teardrop-shape, one-cell parasite. It has three tails at its narrow end. By whipping these tails back and forth the organism can swim about in a brisk, if rather jerky, fashion. As it swims, the host's white blood cells follow in hot pursuit. These blood cells, critical to the human immune defense system, can literally surround a trichomonad and digest it. Presumably, symptoms occur only when the body's defense forces cannot keep up with the numbers of rapidly reproducing trichomonads.

Trichomoniasis is usually easily diagnosed by a case history, pelvic examination, and microscopic examination of a sample of vaginal discharge. Your doctor may take a vaginal Pap smear or trichomonas culture if the diagnosis is unclear.

Metronidazole (Flagyl) is the most effective treatment for trichomoniasis. A single 2000 milligram dose (four 500 milligram tablets taken by mouth all at once) usually works well. If not, the treatment is generally extended over 7 days (either 250 milligrams 3 times daily or 500 milligrams twice daily). Metronidazole gel is usually not recommended, because it works on this type of infection less than 50 percent of the time. To prevent re-infection, your sexual partner must be treated as well.

The use of metronidazole in pregnancy is controversial. Many doctors avoid prescribing this medication at least until the second or third trimesters. If you are pregnant and have trichomoniasis, anti-yeast vaginal creams, which also affect trichomonads, will provide significant relief. If you are unsure whether you are pregnant, you should either have a pregnancy test taken at least 2 weeks after your last intercourse or simply delay starting treatment with metronidazole until your menstrual period begins.

This drug interacts with alcohol, causing abdominal cramps, nausea, and vomiting. Therefore, it is important not to drink alcoholic beverages while taking this medication. If you have active liver disease, certain blood disorders, current or past seizures, or a history of metronidazole allergy, you should avoid using this medication.

As with all STDs, the only certain way to avoid infection is to refrain from all sexual activity. Condoms with a spermicide offer excellent protection for those who remain sexually active. Remember that most men and nearly half of all women with trichomoniasis have no symptoms! Even if your partner is the picture of health, use a condom!

Remember, too, that if you find you have one STD, there's a chance you may have others as well. For more information on this and other STDs, see "Coming to Terms with a Sexually Transmitted Disease."

Whatever you do, if vulvar irritation or itching fails to go away be sure to check with your doctor. Especially in menopausal women, these symptoms could mean the presence of a precancerous condition or a cancer.