The most common sexually transmitted vaginal infection in the U.S., caused by an obligate intracellular parasite, Chlamydia trachomatis. Chlamydial infection is also a major cause of pelvic inflammatory disease, tubal occlusion, infertility, ectopic pregnancy, nongonococcal urethritis, and ophthalmia neonatorum. Asymptomatic chlamydial infection has been implicated in the development of preterm labor and birth in high-risk women. Symptoms include a thin or purulent vaginal discharge, dysuria, and lower abdominal pain. Diagnosis is established by testing for specific monoclonal antibodies. Doxycycline is the drug of choice, except during pregnancy (it damages fetal bone and tooth formation). During pregnancy the infection is treated with erythromycin or azithromycin.