There are two distinct culprits behind head and neck cancer - the long-recognized heavy tobacco and alcohol use as well as a common sexually transmitted virus, researchers said on Tuesday.

The risk factors are so dramatically different in head and neck cancer in people infected with the human papillomavirus, or HPV, that it should be considered a separate disease from cases in which patients are not infected, researchers at Johns Hopkins University in Baltimore said.

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

“These are completely different cancers and we need to view them as such. 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, said in a telephone interview.

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 patient populations different in HPV-positive and HPV-negative head and neck cancer, but 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. It is well known for causing cervical cancer and genital warts.

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

The new study, published in the Journal of the National Cancer Institute, 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.

The people with viral-linked cancer cases had a completely different set of risk factors, including certain sexual behaviors and marijuana use, the researchers found.

Sexual behaviors linked to these patients included increasing numbers of lifetime sex partners including oral sex, and the presence of a sexually transmitted disease, they said.

Gillison said it is possible other behaviors linked with marijuana use could be responsible, but said chemicals in marijuana called cannabinoids could affect the immune system’s ability to clear a viral infection.

By Will Dunham

WASHINGTON (Reuters)