The human papillomavirus (HPV) is a cluster of more than 100 viruses linked to some cancers and other diseases.

It is believed to be a factor in about one in four head-and-neck tumors, especially along the tonsils, according to the American Cancer Society (ACS). It is not known why HPV, which many people have, triggers cancer in some people and not in others.

People with HPV-linked tumors are less likely to be smokers and drinkers. Research suggests that patients with HPV may have a better survival rate, said Dr. Vasu Divi of the University of Michigan.

The ACS estimates that this year 25,800 new cases of oral and related cancers will be diagnosed in the United States and 5,830 people will die from them. They are twice as common in men. A third occur in people younger than 55. Typically they are linked to smoking and alcohol use.

Movie critic Roger Ebert attributes his cancer to HPV, while actor Michael Douglas acknowledges a history of smoking and alcohol.

Many head-and-neck cancers cause no symptoms. the cancer society recommends routine exams of the mouth. some dentists and doctors may use special dyes or lights, but these newer techniques are still being studied.