Chlamydia is the most common STI caused by bacteria, but because it's typically asymptomatic, many people can have it and not know it. (Photo: Getty Images) There are more than 20 types of sexually transmitted infections (STIs), but some are more prevalent than others — namely, chlamydia, which is the most common STI caused by bacteria. (HPV, also known as human papillomavirus, is the most common STI caused by a virus.) In fact, chlamydia trachomatis — the bacteria behind chlamydia infections — is "responsible for the greatest number of sexually transmitted infections" worldwide. In 2020, nearly 1.6 million cases of chlamydia were reported in the U.S, according to the Centers for Disease Control and Prevention (CDC), "making it the most common notifiable sexually transmitted infection in the United States for that year." As Dr. Saahir Khan, infectious disease expert with Keck Medicine of USC, tells Yahoo Life: "Chlamydia has been infecting humans for mille...