Trichomoniasis (aka "trich") is a sexually transmitted disease caused by a flagellated protozoan (Trichomonas vaginalis). It infects about 3 million people in the U.S. each year and is the most common curable STD found in young, sexually-active women. The World Health Organization estimated that in 1996, 170 million people around the planet were infected with this disease.
Generally, men only catch it from women via vaginal sex; the disease can be transmitted between women by vulva-to-vulva contact. The disease may also be transmitted by sharing towels and similar damp items. An infected pregnant woman may infect her baby during birth.
Infection in men generally produces no symptoms, though occasionally there is penile discharge and/or burning during urination or after ejaculation due to inflammation of the urethra.
Half of infected women may also experience no symptoms. If left untreated, the infection can progress to pelvic inflammatory disease. Additionaly, trich infections during pregnancy cause an increased risk of premature membrane ruptures and of premature birth.
Trich infections make both sexes more vulnerable to contracting and transmitting HIV (the virus that causes AIDS) due to the inflammation and tissue damage the protozoan causes.
Infections may go on for years with no symptoms. Women's symptoms include:
Symptoms occur 4-28 days after infection.
The risk of transmission is can be lowered by using condoms; diaphragms provide some protection, but not as much as condoms. A woman's risk of contracting trich increases if she is taking birth control pills, is pregnant, has just finished a menstrual period, or has another STD.
Doctors may spot an otherwise symptomless trich infection during a routine gynecological exam because the disease often causes tiny red ulcers on the vagina or cervix. If a physician or patient suspects an infection, proper diagnosis is made by taking a sample of vaginal discharge and examining it under a microscope for the presence of the protozoans. Diagnosing men is a bit more difficult; the doctor will take a swab of the man's urethra and examine the residue in the lab.
Medication (usually a single dose of the prescription drug metronidazole, sold as Flagyl) is effective for curing the disease. Pregnant women can be given the drug. Both partners should be given treatment, because a symptomless-but-infected man can repeated reinfect his partner. A person undergoing treatment for trich should avoid sex until after the treatment is completed and the symptoms have cleared up.
References: http://www.cdc.gov/nchstp/dstd/Fact_Sheets/FactsTrichomoniasis.htm and http://www.bu.edu/cohis/std/trichmn.htm