Chlamydia is a sexually transmitted disease (STD) that is often unknowingly passed to others through sexual contact, such as vaginal, anal, or oral sex. This is the most frequently reported bacterial STD in the United States, affecting approximately 2.8 million Americans each year. The more sexual partners a person has, the higher the risk of getting infected with chlamydia. Because there are often no symptoms, people who are infected may unknowingly pass the disease on to their sex partners. Treatment for this curable infection generally involves a course of antibiotics.