Head and neck cancer includes tumors in the mouth, tongue, nose, sinuses, throat and lymph nodes in the neck.

"These are completely different cancers.They just happen to occur in the same place. The risk factors didn't appear to overlap at all, and there didn't appear to be any interaction between them," Dr. Maura Gillison, a professor of oncology and epidemiology at Johns Hopkins University in Baltimore said.

More than 35,000 people are diagnosed with head and neck cancer annually in the United States alone. If found early, such cancer may respond well to treatment with surgery, radiation therapy and chemotherapy.

Not only are the head and neck cancer patients different in HPV-positive from HPV-negative, but also the tumors look different under a microscope, Gillison said. People with the viral-linked cancer also tend to respond better to treatment than those not HPV-infected, she added.

HPV is a common sexually transmitted virus, well known for causing cervical cancer and genital warts.

Since 2000, researchers have also known that HPV infection is linked to some cases of head and neck cancer, particularly in the upper throat and back of the tongue.

The new study focused on 240 people diagnosed with head and neck cancer between 2000 and 2006.

Nearly 40 percent of them had an HPV infection. Those patients did not have the well-known risk factors for head and neck cancer -- tobacco smoking, alcohol use and poor oral hygiene, the researchers found.