Chlamydia Chlamydial infection is the most common bacterial sexually transmitted disease in the United States today. The U.S. Centers for Disease Control and Prevention estimates that more than 4 million new cases occur each year. The highest rates of chlamydial infection are in 15 to 19-year old adolescents regardless of demographics or location. Pelvic inflammatory disease (PID), a serious complication of chlamydial infection, has emerged as a major cause of infertility among women of childbearing age. Chlamydial infection is caused by a bacterium, Chlamydial trachomatis, and can be transmitted during vaginal, oral, or ana