Chlamydia is a common sexually transmitted infection (STI) caused by the bacterium Chlamydia trachomatis, which affects the genital tract if acquired through vaginal sex and the mouth or anus if acquired from oral or anal sex. It is spread through unprotected sex with an infected person; it can also be spread from an infected birthing parent to their baby at birth.

Most of the time, chlamydia is a "silent" infection and has few symptoms. However, if left untreated it can cause permanent damage to the woman's reproductive tract. In men, chronic infection can lead to sterility.

For women, pelvic inflammatory disease (PID) is inflammation of the upper genital tract from ascending bacteria from the vagina or cervix. Signs and symptoms include lower abdominal pain, vaginal discharge, abnormal vaginal bleeding, abdominal guarding and rebound, and other signs of peritonitis, adnexal tenderness, cervical motion tenderness, and fever.

Fitz-Hugh-Curtis is a complication of PID causing inflammation of the liver capsule.

Consider inpatient hospitalization if a tubo-ovarian abscess is suspected, there is no response to oral antibiotics, the patient is unable to follow an oral regimen, the patient appears very ill, or other abdominal pathology cannot be ruled out.

For men, chlamydial infection can also cause epididymitis or orchitis, which may manifest as pain or swelling of one or both testicles or discomfort with urination. It may trigger reactive arthritis (Reiter syndrome), a disorder classically characterized by arthritis, conjunctivitis, and urethritis, a few weeks after the primary infection in those individuals who may be predisposed (HLA-B27 positive).

In all cases where a child is being evaluated for sexual abuse, specific screening tests for all STIs should be pursued, even in the absence of specific symptoms.