One of the sexually transmitted infections that I get asked most about is Human Papillomavirus (HPV). Thanks to relentless advertising for Merck's Gardasil vaccine, many people are aware that some strains of HPV are linked to cervical cancer in women, and about 37% of teen girls have received the Gardasil vaccine in the U.S. since it was recommended for routine administration in young women in 2006.

What about men? HPV’s effects aren't limited to cervical cancer—it can also cause genital warts and anal cancer in both men and women, and although anal cancer is rare, incidence has been increasing over the past several decades. In the past, I've talked to straight men who were upset that they may have unwittingly given their girlfriends a virus that causes cervical cancer (there are no routine tests to detect HPV in men), and I've talked to gay and bisexual men who were frustrated that they didn't have the same access to HPV vaccines that women did.

In October 2009, Gardasil was approved for administration in young men, but its effects were seen as largely altruistic. Yes, the vaccine would protect men against genital warts, but in the public health field, warts are not perceived to be as serious a problem as cervical cancer (although many individuals who have had to deal with warts would probably say that their effect is not negligible!). Yes, the vaccine would protect against anal infection among men who have receptive anal sex, but most scientists would consider this a small slice of the population, and since the vaccine is supposed to be given around age 11, before sexual activity has begun, it's unlikely that parents are going to be factoring this into their decision-making. "Hey, just in case your son turns out to be queer, would you like to give him this vaccine?" We're talking about a country where some parents don't even want to give girls the vaccine lest it encourage them to have pre-marital sex.

However, a new study published last month in the Journal of Infectious Diseases demonstrates that anal HPV infection is much more common among heterosexual men than previously thought. While gay and bisexual men are estimated to be 17 times more likely to develop HPV-related anal cancer, there haven't been any good estimates of how many heterosexual men might be at risk.

The study found that 12% of heterosexual men in the US, Brazil and Mexico had an anal HPV infection, and 7% of those with an infection carried a strain related to cancer. By comparison, less than 1% of men in the U.S. are infected with Chlamydia at any given time, and Chlamydia is considered quite common. Some of the men in the study had previously had male sex partners, but the majority had not. Study researchers speculated that HPV could have been transmitted anally to men via their female partner's hands or could have moved from the men's genitals to the anus.

An October 2009 study in the British Medical Journal found that vaccinating men with Gardasil was not a cost-effective public health measure, meaning that the benefits obtained in preventing disease were not greater than the costs incurred in administering the vaccine. However, this study looked only at preventing cervical cancer and genital warts, not at anal cancer. Our knowledge of HPV changes almost daily and this latest study is more evidence of how much we do not know about sexually transmitted infections and how many assumptions we make based on a person's sexual orientation. HPV vaccination for men may be more viable than it appears.