Chlamydia is a bacterial infection of the urethra in men and of the urethra, the cervix, or the upper reproductive organs (or all three) in women. Chlamydia is transmitted by sexual contact and in many countries is the most common sexually transmitted infection (STI).

Chlamydia can also infect the rectum and the lining of the eyelids (conjunctiva).

Chlamydia may cause discharge from the vagina or penis and painful urination. But it often does not cause any symptoms, and the person may not know that he or she is infected. If left untreated, chlamydia can cause pelvic inflammatory disease. It may lead to infertility in women and men.

Chlamydia can infect the eyes if they come in contact with genital secretions from an infected person. The bacteria can also be spread to a baby during birth if the mother has the infection. This causes problems with the baby's eyes or a lung infection (pneumonia). Antibiotic eyedrops are commonly given to babies just after birth to prevent eye infections.

Chlamydia can be cured with antibiotics. Both sex partners need to be treated at the same time.