Chlamydia is the most common bacterial sexually transmitted infection (STI) in the United States. Chlamydia is caused by the bacterium Chlamydia trachomatis.
Even though symptoms of chlamydia can often be mild or absent, serious potential complications can cause irreversible damage, including infertility.
Chlamydia can be transmitted during vaginal, anal or oral sex. Chlamydia can also be passed from an infected mother to the baby during vaginal childbirth.
Any sexually active person can be infected with chlamydia. The greater the number of sex partners, the greater the risk of infection. Because the cervix (opening to the uterus) of teenage girls and young women is not fully matured, they are at particularly high risk for infection if sexually active.
Since chlamydia can be transmitted by oral or anal sex, men who have sex with men are also at risk for chlamydial infection.
If untreated, chlamydia can progress to serious reproductive and other health problems with both short-term and long-term consequences.
In women, untreated infection can spread into the uterus or fallopian tubes and cause pelvic inflammatory disease (PID). This happens in up to 40 percent of women with untreated chlamydia.
PID can cause permanent damage to the fallopian tubes, uterus and surrounding tissues. The damage can lead to chronic pelvic pain, infertility and potentially fatal ectopic pregnancy (pregnancy outside the uterus). Women infected with chlamydia are up to five times more likely to become infected with HIV, if exposed.
To help prevent the serious consequences of chlamydia, screening at least annually for chlamydia is recommended for all sexually active women age 25 years and younger.
In men, infection sometimes spreads to the epididymis (a tube that carries sperm from the testis), causing pain, fever and, rarely, sterility.
Last Reviewed: September 2019
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