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There’s a virus that almost everyone gets at some point in their lives.

It can cause cancer, and rates of the cancers it causes are growing among men.

Paul Cheung / NBC News

The virus is the human papillomavirus or HPV, and it’s the main cause of cervical cancer, anal cancer and, more recently, mouth and throat cancer.

It’s passed mostly via sex, but doctors aren’t talking about that to their patients. Why not?

It’s because there is a vaccine to prevent HPV infection. But for it to work best, people have to get it long before they could ever be infected. That means vaccinating 11- and 12-year-olds, and most parents get very squeamish about the thought of sex and kids that age.

“There are some taboo subjects but the fact is that almost every human being is going to get HPV at some point in their life through normal, intimate human activity,” said Dr. Lois Ramondetta, chief of gynecologic oncology at the MD Anderson Cancer Center in Houston.

Related: There's a Silent Epidemic of Cancer Among Men

HPV is the main cause of cervical cancer, once a leading cancer among women. It still causes 11,000 cases of cervical cancer a year in the U.S., but screening via pap smears and, more lately, HPV tests, has cut the rate of new cases and of deaths dramatically.

“There’s nothing that men and women can do to prevent oral cancer."

The CDC estimates that HPV causes 31,500 cases of cancer a year. Most are in women, but more head and neck cancers are caused in men than in women.

Earlier this week, researchers reported that a startling number of men are infected with HPV in the mouth and throat — 11 percent of all men tested between 2011 and 2014. They are at risk of developing oral cancer.

This cancer mostly starts to show up in middle age, and men understandably want to know why they got it. Oral sex appears to be the cause — probably dating back to when they were in their late teens or early 20s.

'Every human being is at risk'

But it turns out there’s not much people can do to discover they’re infected.

“There is no test to find out a person’s ‘HPV status’,” the Centers for Disease Control and Prevention says. “Also, there is no approved HPV test to find HPV in the mouth or throat.”

And there’s no way to go back and undo whatever caused the infection that has caused cancer decades later.

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“We don’t know how it goes from being an infection to causing the cancer,” Ramondetta said. “And we don’t know why the virus doesn’t go away in some people.”

Smoking either tobacco or marijuana seems to increase the risk of a long-lasting HPV infection, as does having multiple sex partners.

“But really every human being is at risk,” Ramondetta said.

“There’s nothing that men and women can do to prevent oral cancer,” said Dr. Noel Brewer, who studies health behavior at the University of North Carolina. “There’s no screening that they can get. There’s no behavior changes they can make to lower their risk, other than to stop smoking or drinking.”

And once someone has started having sexual contact, the odds are they’re infected with HPV. At that point, it’s too late for the vaccine.

Related: Pre-Teens Need Just Two Doses of HPV Vaccine

"I wish there had been a vaccine when I was a child," said Sandy Wexler, a 63-year-old retired nurse living in Houston who has been successfully treated for oropharyngeal cancer.

"A parent never wants to think that their kid is going to have sex, but they are going to. We need to vaccinate them before they are exposed," added Wexler, whose cancer was detected by an alert dentist. She pointed out that even if a person is completely monogamous, they can never be certain their partner has been.

Since there’s not much adults can do to protect themselves, doctors and public health professionals have decided to make the focus getting kids vaccinated, instead.

“There is a lot of hope the HPV vaccine will help to prevent oral cancers... and turn around this epidemic,” Brewer said.

Related: Too Few Kids Get HPV Vaccine

“There’s been some research done that found greater success in getting 11- to 12-year-olds vaccinated if they really do put the focus on the fact that the HPV vaccine is important because it prevents cancer,” said Glen Nowak, director of the Center for Health and Risk Communication at the University of Georgia.

“The primary focus these days is on the benefits of the vaccine,” he added. For example, parents are reminded they use bike helmets and seat belts to protect their children, and vaccines also protect them.

“So they don’t place so much emphasis on how HPV is transmitted,” Nowak said.

"This is not about sex. This is about cancer.”

And at first, doctors thought having an HPV vaccine might open the door to having that cringe-making sex talk with boys and girls.

“We have realized since then that talking about sex is such a barrier to the discussion,” said Brewer.

Parents get upset, and often decide it’s too early for their kids to be vaccinated against a sexually transmitted disease, even though the research clearly shows that getting the first dose by age 11 or 12 really does protect kids better. And delaying vaccination often means the child never gets vaccinated at all.

Related: HPV-Related Cancers Epidemic in Men

It can bother doctors, too.

“About a third of physicians say talking about HPV is uncomfortable,” Brewer said.

“Many physicians don’t want to talk to an 11-year-old girl about sex. Parents don’t want to talk to an 11-year-old girl about sex. And, honestly, an 11-year-old girl wants to talk about sex with everybody but the two people in that room.”

The undated handout photo shows an electron microscope image of human papilloma viruses. German Cancer Research Center via AP file

More and more men and women are likely to develop oral cancer in the coming years — a lot of people are too old to have been vaccinated.

“About 79 million Americans are currently infected with HPV,” the CDC says. “About 14 million people become newly infected each year. HPV is so common that most sexually active men and women will get at least one type of HPV at some point in their lives.”

So, Ramondetta said, the enemy is not sex. It’s the virus.

“This should not be about sex. Pregnancy is sexually transmitted. Sex is a normal human activity. This is not about sex. This is about cancer.”