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In 2015, documented cases of sexually transmitted diseases in the US hit the highest number ever in the Centers for Disease Control and Prevention’s records going back to 1941, according to a new report by the agency. It’s the second year in a row with historically high STD levels. However, because only three STDs are routinely reported to the CDC—chlamydia, gonorrhea, and syphilis—the new data may be just a glimpse of the nationwide problem.

Among the three nationally reported STDs, chlamydia had the highest total number of cases, hitting more than 1.5 million in 2015. That total is a 5.9 percent increase from that of 2014. Syphilis saw the largest jump, with a 19 percent increase from 2014 cases, bringing the 2015 total to nearly 24,000. Gonorrhea followed with a 12.8 percent increase, reaching nearly 400,000 cases.

Americans most affected by the STD rise are young people and gay and bisexual men, the agency found. Two-thirds of chlamydia cases and half of gonorrhea cases were among Americans aged 15 to 24. Men who have sex with men were largely behind the rises in gonorrhea and syphilis.