Insurance claims for head and neck cancers increased 61 percent over the last five years, according to a report from FAIR Health, and scientists think oral sex might be to blame. It’s especially bad for men—they made up three out of four new cases during that time period.

So what’s happening? One recent study found men who recently performed oral sex had a higher likelihood of having an oral HPV infection, which could lead to head and neck cancers. The more sexual partners a man has, the higher his risk. This wasn’t the case for women, likely because they commonly build up immunity to HPV, though scientists aren’t sure what causes this resistance.

Smoking is still the leading cause of head and neck cancers, but as fewer people smoke cigarettes, researchers don’t believe they are responsible for the dramatic increase in cancer cases.

The takeaway? Steer clear of cigarettes and get the HPV vaccine, just to be safe.