Chlamydia is a sexually transmitted disease (STD) that is caused by a type of bacteria called Chlamydia trachomatis. It is the most frequently reported bacterial STD in the United States. Chlamydia is often known as a "silent" disease, because most people who are infected show no symptoms. When symptoms of chlamydia do develop, they may include abnormal discharge from the penis or vagina and a burning sensation when urinating. Treatment for chlamydia generally involves a course of antibiotics. If left untreated, the disease may lead to serious complications.