Sexually transmitted infections spread from 1 person to another during sexual relations. Chlamydia trachomatis is a bacterium that causes a common sexually transmitted infection (chlamydia). Chlamydia can cause discharge from the vagina or penis, pain with urination, or abdominal pain. However, some people with chlamydia have no symptoms. Treatment with antibiotics relieves symptoms, stops the spread of infection, and prevents complications of the infection. In women, chlamydia can cause difficulties in becoming pregnant (infertility). Infections in pregnant women can lead to premature birth and other problems in newborns.