National data from the CDC indicate that there were more than 1 million reported cases of chlamydia, more than 300,000 reported cases of gonorrhea, and nearly 18,000 reported cases of syphilis in 2013. The report states that patients aged 15 to 24 years accounted for the majority of chlamydia and gonorrhea cases, while men who have sex with men accounted for 75% of syphilis cases.

Source: Centers for Disease Control & Prevention.