Here's a crucial message for teens: Oral sex carries many of the same risks as vaginal sex, including human papilloma virus, or HPV. And HPV may now be overtaking tobacco as the leading cause of oral cancers in America in people under age 50.

"Adolescents don’t think oral sex is something to worry about," said Bonnie Halpern-Felsher professor of pediatrics at the University of California, San Francisco. "They view it as a way to have intimacy without having 'sex.'"

Halpern-Felsher and other researchers presented the latest information about the risks of contracting an HPV infection Sunday at the American Association for the Advancement of Science annual meeting.

The latest data suggest that 64% of oropharynx cancers - growing in the middle part of the throat - in the United States are caused by HPV, which is more than tobacco causes, said Maura Gillison of Ohio State University. And as the number of partners on whom you have performed oral sex goes up, the risk of oropharnyx cancer goes up.

About 37,000 people per year receive a diagnosis of oral cancer, according to the Oral Cancer Foundation.

Just about everyone has had at least one of the 130 strains of HPV. The vaccines currently available, Gardasil and Cervarix, only protect against a few of them. But not all of the strains are cancer-causing. Certain types cause warts on the hands and feet that are benign.

About 5% of cancers worldwide are caused by HPV, and some turn up in some surprising places. A University of Washington study found that some men carry HPV 26 under their fingernails, which can lead to a form of cancer called digital squamous cell carcinoma. Proper hand-washing can help prevent this from happening, said Dr. Diane Harper, leading HPV researcher at the University of Missouri in Kansas City.

But this isn't nearly as common as HPV causing cancer in other areas of the body, such as the oral cavity and the cervix.

In the countries that have cervical cancer screening, the prevalence of cervical cancer is five times lower than in other countries, indicating that the testing is effective, Harper said.

Why HPV causes cancer in some people and not others is still mysterious. Studies of the cervix have found that 70% of infections resolve by themselves within one year, and 90% within two years. It's that remaining 10% that actually turn into more serious infections, and 5% lead to treatable precancerous lesions, Harper said.

Two well-established mechanisms of prevention in terms of sexually transmitted HPV are condom usage and circumcision, although neither completely eliminates the risk, Harper said.

A large ongoing study called HITCH is examining questions of HPV transmission and infection in greater detail. So far, it's found that couples can "ping-pong" HPV back and forth to each other, which is one reason that the virus may take so long to clear naturally.

As for getting HPV from kissing, that's not clear, and there isn't enough data to say anything about it yet, Harper said.

It's very hard to get teens to listen to abstinence messages about oral sex, or to get them to use any kind of barrier method for these behaviors, Halpern-Felsher said. And since any risk factor under 50% sounds low to a very young person, throwing these precise statistics at them most likely won't make a difference.

But parents should have honest conversations with their teenagers about oral sex, Halpern-Felsher said. Tell them that the consequences of HPV may not happen right away, and while the risks may not be huge, they are significant. Potential long-term outcomes of cancer are quite concerning.