Sexually transmitted diseases are infections that spread from one person to another during sexual relations. Chlamydia trachomatis is a bacterium causing a common sexually transmitted infection (chlamydia) in the United States. 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 antibiotic drugs is important to get rid of symptoms, stop the spread of infection to others, and prevent complications of the infections. In women, chlamydia can cause difficulties in becoming pregnant (infertility). Infections in pregnant women can lead to premature birth and other problems in newborns.