Chlamydia is a sexually transmitted bacterial infection caused by Chlamydia trachomatis. It can spread through vaginal, anal or oral sex. Symptoms include painful urination, increased vaginal discharge and discharge from the penis. Asymptomatic infections are common. Without antibiotic treatment, chlamydia can lead to infertility or pass from mother to infant during birth. It can be prevented through safer sex practices such as condom use and is treated with antibiotics.