Chlamydia: The agent of a sexually transmitted disease, a type of bacteria found in the cervix, urethra, throat, or rectum that acts very much like gonorrhea in the way it is spread, the symptoms it produces, and its long-term consequences. Chlamydia is destructive to the Fallopian tubes, causing infertility, tubal pregnancy, and severe pelvic infection. It is common for infected women to have no symptoms. Chlamydia is associated with an increased incidence of preterm births. Also, an infant can acquire the disease during passage through the birth canal, leading to eye problems or pneumonia. Chlamydia is one of the reasons newborns are routinely treated with antibiotic eyedrops. Chlamydia can also cause inflammation of the urethra, epididymis, and rectum in men. A chronic form of arthritis, called reactive arthritis, can develop after chlamydia infection.