One of the most common sexually transmitted diseases around is chlamydia. According to the Centers for Disease Control (CDC), in 2002, 834,555 infections were reported in the United States. Yet, the numbers are suspected to be even higher than that. Under-reporting is significant because most people don't realize they have the disease and unwittingly spread it to their sexual partners.
If symptoms of chlamydia do appear, they can include a burning sensation when urinating and an abnormal discharge from the vagina or penis. The infection is easily treated with antibiotics, and prompt treatment is essential to prevent potentially serious complications.
(Click Chlamydia for more information on this sexually transmitted disease, including what causes it, how it is diagnosed, and the special risks it presents during pregnancy.)