The next time you think about going down on a woman, think about this: She is almost certain to have been infected at some point with a virus that could, years from now, give you throat cancer.

The most common sexually transmitted disease in the U.S., human papillomavirus, or HPV, has been blamed for a recent, rapid increase in the incidence of throat cancer – a disease that used to be rare in people who didn't drink or smoke excessively. According to data from 2004, the most recent available, rates of HPV-related throat cancer had risen 225 percent in the previous 16 years, with men suffering the most cases. Researchers point to the increasing popularity of oral sex – often seen as safer than intercourse – among heterosexual couples, a trend that may soon lead to more male fatalities in industrialized nations from HPV-related infections than female ones – a surprising turnaround after decades when women suffered higher death rates from the virus, which also causes cervical cancer through vaginal sex.

UPDATE: HPV Prevention for Men

Most people who contract HPV get rid of the virus within a few years without side effects or complications, and the number of men infected with HPV who actually develop cancer is still very small. The bad news, however, is that researchers believe there is virtually nothing a heterosexual man with a normal sex life can do to avoid HPV infection. Virgins who have done nothing but open-mouthed kissing have been found to be infected. One study even discovered that kissing may increase your risk of oral infection more than having intercourse. How about monogamy? Sorry, but that one's out, too. A study of coeds at the University of Washington found that half of women who had had only one sexual partner were infected with the virus after three years of partnership. Even fooling around can be risky, since skin under the fingernails can contain the virus. Condoms are helpful, but HPV can attach to so many different surfaces that even they are not foolproof.

A large part of the problem is HPV's universality. The Centers for Disease Control and Prevention approximates that 80 percent of women in the U.S. have been exposed to the virus by age 50, an estimate that some researchers think is low. No one is quite sure what the exposure rate among men is, although guys tend to become infected more readily than women: Studies of college students found active infections in half of women and nearly two-thirds of men. "If you're sexually active, in all probability, you have already been exposed," said Dr. Maura L. Gillison, a professor of medicine at Ohio State University's comprehensive cancer center.

While there are more than 100 strains of the HPV virus, most oral cancers result from HPV-16. Those who test positive for HPV-16 are 14 times as likely to develop oral cancer as those who do not. (There are at least seven other strains known to cause cancers, including cervical, anal, penile, and vulvar, as well as six more suspected of doing so.)

Unlike cervical cancer, which can be detected with a Pap smear, there is no test that can easily identify HPV-related throat cancer. By the time those with the disease become aware that they're sick, cancer has often spread to their lymph nodes. Surgery can be disfiguring, and chemotherapy and radiation are exhausting and debilitating. And while HPV-related throat cancers are generally more curable than those that result from smoking or alcohol use, 40 percent of the 36,000 people diagnosed each year with oral cancer will die from the disease within five years. Between 1992 and 2001 – in the most recent data available – oral cancer ranked as the seventh most common cancer among men in the U.S.

In 2007, Kevin McConnell, 51, of Annapolis, Maryland, developed what he thought was a terrible earache. After the pain spread to his neck and tongue, he saw a doctor who referred him to a specialist. Although the specialist found no evidence of cancer, he sent him for two brain scans, both of which were negative. Three weeks later, McConnell developed a lesion on his tongue that spurred him to see an expert at Johns Hopkins, who told him he had a 3.5-centimeter tumor that was cancerous and life-threatening. After seven weeks of intensive radiation and treatments, McConnell has recovered and is in remission, but he says he gets mixed reactions when he tells women he dates that he had oral cancer as a result of an HPV infection. "I've had some people freak out about it and say they don't want to date me," he says, pointing out – accurately – that most of these women have probably already had the virus themselves.

Although there's little you can do to decrease the possibility of contracting HPV, researchers have identified risk factors. Losing your virginity at an early age and having many sexual partners boost your chances; so do tobacco use and a history of genital warts. But many mysteries remain, including why men who use marijuana seem to be at greater risk for HPV-related oral cancers and what role race might play. A 2009 study, for ­example, found that white men are almost nine times as likely as black men to suffer HPV-related oral cancers. Whether this disparity is owing to a difference in sexual behavior is unknown.

Another mystery is why men are three times as likely as women to develop HPV-related oral cancer, although many researchers suspect hormonal differences account for gender discrepancies. Men are more likely than women to have active HPV infections, but they also tend to have lower immune responses to the virus. What's more, as women age, they are less likely to harbor active infections, while older men seem to be just as able as younger men to contract the virus. Another theory has to do with sexual anatomy: Vaginas may spread HPV into the mouth more readily than ­penises, since vaginas usually contain more fluids, in which HPV particles can reside, than the penile shaft or scrotum, the surfaces most involved in oral sex. (Whether HPV resides in semen is still unknown.)

A more worrisome mystery is whether there is a link between HPV and heart disease. A recent study found that women with HPV are two to three times as likely as uninfected women to have had a heart attack or stroke at some point in their lives. It's not clear whether HPV increases the risk of cardiovascular problems or whether women with heart ailments are more likely to contract HPV.

While there is no treatment for HPV infection, two vaccines – Merck's Gardasil and GlaxoSmithKline's Cervarix – protect against HPV-16, the strain associated with oral cancer. But the vaccines have been proved to work only in those not yet exposed to the virus, and only Gardasil is approved for use in males. In 2006, the CDC recommended that girls and young women under age 26 be vaccinated; in October of this year, the agency also advised that all 11- and 12-year-old boys be vaccinated, as well as males ages 13 through 21 who have not had the vaccine's routine course of three shots (which very few have had). "The reason vaccination is important is that we can't distinguish between people who will develop cancer as a result of an HPV infection and those who will not," Gillison says.

Although most major health organizations see vaccination as a medical necessity, it's considered controversial, largely because the cancer it prevents results from sexual activity. The vaccines are also expensive: Gardasil usually costs more than $300 for all three shots. In part for these reasons, fewer than half of girls ages 13 to 17 have received at least one shot, and fewer than a third have received all three required doses. Only about one percent of boys have received the HPV vaccine.

The vaccines became a source of contention among Republican presidential candidates after some criticized Texas governor Rick Perry for trying to mandate that all girls in his state be vaccinated. Representative Michele Bachmann falsely suggested in televised remarks that the vaccine causes mental retardation.

But many men, including Bruce Arbaugh, 56, of Reynoldsburg, Ohio, believe in vaccination. Two years ago Arbaugh noticed a painless bump on his neck that he thought was from a bee sting. After two weeks it had almost disappeared from view until a bike ride gave him a stiff neck. When his wife, Diane, rubbed his neck, she felt a lump the size of a golf ball. Arbaugh went to see a nurse who, thinking he had an infection, prescribed two different courses of antibiotics over two weeks to no effect. X-rays and other tests were inconclusive until a second biopsy finally showed cancer. A surgeon removed 30 lymph nodes under Arbaugh's jaw, 12 of which were cancerous. Radiation and drugs eventually rid him of the disease, although he had to have most of his back teeth removed.

To help prevent cases like Arbaugh's, Dr. William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University, says vaccinating boys against HPV is imperative because it prevents "a notable number of cancers," many of which are diagnosed too late and require ineffective and often disfiguring treatment. "Is it worth it to prevent those cancers?" he asks rhetorically. "Oh, God, yes."

See also: How to Minimize HPV and Cancer Risk