Chlamydia-- Chlamydia is transmitted mainly during vaginal or anal sex (unlikely, but possible, during oral sex) Chlamydia is especially dangerous because in women it very often has no symptoms, but if left untreated, it can leave a woman infertile. The disease can cause scarring of the fallopian tubes, sterility, infertility, ectopic pregnancy, or chronic pelvic pain. Burning during urination or an unusual discharge may be the only symptoms. Men with chlamydia usually do experience this burning. It is important that infected men inform all of their recent partners. Chlamydia is easily treated with antibiotics. How Common? 4 million new cases are reported a year; 2.6 million of these are women. Help: CDC National STD Hotline, 800-227-8922.