They say the human papilloma virus spread during unprotected sex is to blame for a disturbing rise in potentially deadly oral cancers in the last few decades.
Cancers of the mouth and oropharynx - the top of the throat - used to be mainly diagnosed in older men who drink or smoke. But increasingly, it is being seen in younger men.
Prof Maura Gillison of Ohio State University in Columbus said the sexually transmitted HPV was a bigger cause of some oral cancers than tobacco.
"What is most strongly linked to oral HPV infection is the number of sexual partners someone has had in their lifetimes, in particular the number of individuals on whom they have performed oral sex," the Daily Mail quoted illison as saying.
"The higher the number of partners that you've had, the greater the odds that you'd have an oral infection," Gillison added.
The finding was presented at the American Association for the Advancement of Science conference in Washington CD.