Chlamydia is caused by Chlamydia trachomatis bacteria. It is one of the most common STIs in the world.

The majority of chlamydia infections are among people 15 to 24 years old and it is more common among women than men.

Chlamydia can affect the cervix and urethra, and occasionally the rectum, throat, and eyes. It can also cause another form of STI called lymphogranuloma venerum, also known as LGV or venereal disease.