Actor Michael Douglas says that his throat cancer was caused by a virus he contracted by performing oral sex on women — a claim that a doctor called dubious. The Guardian newspaper published an interview Monday in which Douglas blamed cunnilingus for the grave malady that was diagnosed in 2010. The 68-year-old Douglas has been free of cancer for more than two years after receiving extensive chemotherapy and has returned to acting. Douglas has starred in many movies, including "Basic Instinct" and "Fatal Attraction." He is currently appearing in an acclaimed biopic about Liberace. He told the newspaper his cancer had been caused by having contracted the human papillomavirus (HPV), which he said led to cancer. "It's a sexually transmitted disease," Douglas said of the virus. Dr. Michael Brady, who specializes in sexual health as medical director of the Terrence Higgins Trust, told The Associated Press that oral sex could have contributed to Douglas's cancer but that it was difficult to pinpoint a single cause. He pointed out that Douglas had been a smoker and a drinker, two factors that he said are the most common causes of oral cancer. "There are often a number of factors, genetic, environmental, viral, that could be playing a role," he said. Brady said there are hundreds of different types of HPV virus and that in most cases it is not harmful. The risk is so low, he said, that he does not believe people should worry or change their sexual practices. Doctors estimate up to about one third of mouth and throat cancers are HPV-related and that the types of HPV found in the mouth are likely to have been caused by oral sex. Still, most high-risk HPV types are linked to genital cancers. For some oral cancers, certain sexual behaviors, like having oral sex with four or more people in your lifetime, can raise the risk, according to Britain's Department of Health. Oral cancers caused by HPV are most common in straight men in their 40s and 50s. Douglas is married to the actress Catherine Zeta-Jones.