Chlamydia is the most commonly reported sexually transmitted disease in the United States, infecting nearly 3 million Americans annually. It is caused by the bacterium Chlamydia trachomatis. Chlamydia can be transmitted through vaginal, anal, or oral sex. Many people do not show symptoms, though some people experience abnormal discharge or burning during urination. It is treated with antibiotics. Left untreated, chlamydia can cause serious complications for both women and men.