Chlamydia is a sexually transmitted disease (STD) that is caused by bacteria called Chlamydia trachomatis. The bacteria can be transmitted during vaginal, anal, or oral sex, with an infected person. The more sexual partners a person has, the higher the risk of getting infected with chlamydia. Women can get reinfected with the STD if their partners are not treated at the same time. As a result, women are at high risk for serious reproductive health complications associated with the disease, such as infertility.