Eleven million American men are infected with oral human papillomavirus (HPV), which can lead to cancers of the head, neck and throat, a new study reports.

That equates to 1 in 9 U.S. males aged 18 to 69. And infection is most likely for those who have had multiple oral sexual partners, are gay or bisexual, or who also have genital HPV infection, a team of U.S. researchers found.

The most common cancer caused by the sexually transmitted virus is oropharyngeal squamous cell carcinoma, a head and neck cancer that's far more common in men than women, according to the study.

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"The incidence of this cancer has increased 300 percent in the last 20 years," said lead researcher Ashish Deshmukh. He's a research assistant professor at the University of Florida College of Public Health and Health Professions.

Deshmukh and colleagues used 2011-2014 data from the U.S. National Health and Nutrition Examination Survey.

They found that nearly 12 percent of men and about 3 percent of women were infected with oral HPV.

Nearly 2 million men had high-risk HPV 16, a strain that causes most of the cancers, Deshmukh said. This type was six times more common in men than women.

Although an effective HPV vaccine exists for both boys and girls, the number of boys getting their shots remains low. Also, many at-risk males are older than 26 and don't qualify for the vaccine -- or have already been exposed to the virus, the researchers noted.

The HPV vaccine is recommended before the start of sexual activity. All kids 11 or 12 should get two shots six to 12 months apart, the U.S. Centers for Disease Control and Prevention says.

In 2014, about 57 percent of girls but only 35 percent of boys had been vaccinated, according to the CDC.

"We've got to vaccinate young boys, because vaccine has the potential to decrease cancer risk," Deshmukh said.

Even if all young boys are vaccinated, however, it will be years before a significant decrease in head and neck cancers is seen, he said.

"In the short term, we need to find alternate prevention methods, for example, screening people and identifying precancerous lesions that can be treated," Deshmukh said.

The report was published online Oct. 16 in the Annals of Internal Medicine.

According to 2013-2014 CDC research, more than 45 percent of men were infected with genital HPV, which is more common than the oral type. At the same time, about 40 percent of women carried genital HPV.

Genital HPV can cause cancer of the anus, penis and vagina. Vaginal HPV causes about 70 percent of all cases of cervical cancer, the CDC says.

One specialist said a lot of unknowns surround oral HPV.

"The prevalence of oral HPV is much lower than genital HPV, and we don't understand that," said Patti Gravitt, a professor in the department of global health at George Washington University in Washington, D.C.

It's also not clear why men have more oral HPV than women, she said. Moreover, rates of oral HPV are higher in younger and older people, and that, too, is not understood, said Gravitt, who wrote an editorial accompanying the study.

Fortunately, we have a very effective vaccine," she said.

Unfortunately, many doctors aren't recommending the vaccine, Gravitt said.

HPV vaccine prevents cancer, and "the data are remarkable," she said. "Rarely in a vaccine have you seen such strong effectiveness. ... It's safe," Gravitt said. "We should be doing a better job of protecting people from HPV."