Chlamydia infection, often simply known as Chlamydia, is a sexually transmitted infection caused by the bacterium Chlamydia trachomatis. Most people who are infected have no symptoms. When symptoms do develop this can take a few weeks following infection to occur. Symptoms in women may include vaginal discharge or burning with urination. Symptoms in men may include discharge from the penis, burning with urination, or pain and swelling of one or both testicles. The infection can spread to the upper genital tract in women causing pelvic inflammatory disease which may result in future infertility or ectopic pregnancy.

Chlamydia can be spread during vaginal, anal, or oral sex, and can be passed from an infected mother to her baby during childbirth.

Prevention is by not having sex, using condoms, or having sex with only one other person, who is not infected. Chlamydia can be cured by antibiotics with typically either azithromycin or doxycycline being used. Erythromycin or azithromycin is recommended in babies and during pregnancy. Sexual partners should also be treated and the infected people advised not to have sex for seven days and until symptom free. Gonorrhea, syphilis, and HIV should be tested for in those who have been infected. Following treatment people should be tested again after three months.

Chlamydia is one of the most common sexually transmitted infections worldwide affecting about 4.2% of women and 2.7% of men.