Chlamydia is a common sexually transmitted disease (STD) caused by the bacterium, Chlamydia trachomatis. Chlamydia is a major infectious cause of human eye and genital disease. C. trachomatis is naturally found living only inside human cells and is one of the most common sexually transmitted infections in people worldwide. Chlamydia can be transmitted during vaginal, anal, or oral sex, and can be passed from an infected mother to her baby during vaginal childbirth. Many people with Chlamydia exhibit no symptoms of infection. Between half and three-quarters of all women who have chlamydia have no symptoms and do not know that they are infected. If untreated, chlamydial infections can cause serious reproductive and other health problems with both short-term and long-term consequences. Chlamydia is easily treated with antibiotics.