Chlamydia trachomatis is a common sexually transmitted infection (STI) caused by bacteria. Many people with chlamydia may not know they have it because it often doesn't cause noticeable symptoms, such as genital pain or discharge from the vagina or penis. While it primarily affects young women, chlamydia can occur in both men and women of all ages. It is easy to treat, but if left untreated, it can lead to more serious health issues.