Chlamydia is a common sexually transmitted infection caused by the bacterium Chlamydia trachomatis. It often has no symptoms but can cause pelvic inflammatory disease and infertility if left untreated. It is transmitted through vaginal, anal, or oral sex. Diagnosis involves antigen detection tests of swabs from the genital area or urine samples. Treatment is usually a single dose of azithromycin or one week of doxycycline antibiotics. Partners must also be treated to prevent reinfection.