Chlamydia is a common sexually transmitted infection (STI) caused by the small gram-negative obligate intracellular organism Chlamydia trachomatis. There are 18 serologically variant strains. Serovars D through K cause genital tract infections and will be discussed herein; serovars L1-L3 are the cause of lymphogranuloma venereum. Chlamydiae may infect the genital tract if acquired through unprotected vaginal sex, or the mouth or anus if acquired from oral or anal sex. Chlamydia can also be spread from an infected mother to their baby at birth.

Most of the time, chlamydia is a "silent" infection and has few symptoms. In women, infection may manifest as urethritis, vaginitis, or cervicitis, leading to dysuria, vaginal bleeding, cervical discharge, or abdominal pain.

Chlamydial infection is the most frequently reported bacterial infectious disease in the United States, and prevalence is highest among persons younger than 25 years with new or multiple sexual partners, particularly if protection with condoms is inconsistent. Re-infection happens easily if a sex partner is not treated. Chlamydial infection is a risk factor for other STIs and can increase the risk of acquiring HIV by increasing inflammation of the genital mucosa. There is also an association with the development of reactive arthritis (Reiter syndrome).

Chlamydial infections are common in both sexes, but serious complications are more commonly seen in women. Such complications may include pelvic inflammatory disease (PID) and/or tubo-ovarian abscess, Fitz-Hugh-Curtis syndrome (retrograde spread leads to frank peritonitis and perihepatitis), and septic abortion. The sequelae of PID include infertility and an increased risk of ectopic pregnancy. Pelvic or lower abdominal pain is a marker for such an ascending infection.