Chlamydia is a common sexually transmitted disease caused by bacteria. Most people have no symptoms, so it often goes undiagnosed. It can be transmitted through vaginal, anal, or oral sex. If left untreated, it can cause pelvic inflammatory disease and infertility in women. Symptoms may include abnormal discharge or burning during urination. Diagnosis is made through laboratory tests of samples from infected sites or urine. Treatment involves antibiotics to cure the infection.