CHLAMYDIA - Infection caused by Chlamydia trachomatis are the most common bacterial sexually transmitted infections in the U.S., with an calculated annual incidence of 4.5 million cases. Chlamydia can cause a spectrum of infections including urethritis, epididymitis, and proctitis in men; cervicitis, salpingitis, and acute urethral syndrome in women; and conjunctivitis and pneumonia in newborn infants.