With human papillomavirus, girls and women have been getting all the attention.
Parents across the United States have rushed to have their daughters vaccinated against the virus. States are wrestling with whether to require adolescents be vaccinated.
And recent research found that HPV infection rates among girls and women are higher than previously thought - more than one-quarter of females ages 14 to 59.
Now the attention is turning to boys and men.
As many as 60 percent of men ages 18 to 70 are infected with HPV, according to data not yet published, raising the question of whether the new vaccine will be effective unless men, not just women, are immunized.
Several studies are under way to better understand the virus in males and whether the new HPV vaccine, Gardasil, also will work for them.
"With any transmittable disease, you want to understand the entire cycle of how things spread," says Thomas Broker, an HPV expert and professor of biochemical and molecular genetics at the University of Alabama, Birmingham. "With HPV, men are clearly part of that equation."
Human papillomavirus is best known for causing cervical cancer, with about 9,700 cases diagnosed in women in the United States each year.
Gardasil, a three-shot regimen, was approved last year for girls and women ages 9 to 26. It protects against four strains of the HPV virus that are most likely to cause cervical cancer and genital warts in women.
But much less is known about the consequences of HPV infection in men.
"We know they transmit it to women, but what is the rate of transmission?" says Anna Giuliano, a researcher at the H. Lee Moffitt Cancer Center and Research Institute in Tampa, Fla., who is leading three government-funded studies on HPV infection in men. She is also a paid speaker for Merck, the maker of Gardasil.
Several studies are attempting to address this and other questions. New data show that HPV infection is quite common in men of all ages, while the highest rates of infection in women tend to occur in the early 20s, and then again among women in their 40s and 50s.
"We're seeing a really high prevalence in men, and we see little change in prevalence across the age span," says Giuliano, who found the 60 percent prevalence rate in a study to be published this spring in the journal Cancer Epidemiology Biomarkers & Prevention. "We need to know if women in their 40s and 50s are acquiring new infections from their partners."
HPV infection isn't inconsequential in men. Certain strains of the virus are known to cause genital warts in men as well as women.
And those infections are estimated to be the cause of about half of all anal, penile, vulvar and vaginal cancers and about 20 percent of the cause of all oral cancers, says Dr. Dean Blumberg, an associate professor of pediatric infectious disease at the University of California, Davis.
About 28,000 Americans are diagnosed with oral cancers each year, and about 4,650 are diagnosed with anal cancer. Penile cancer affects about 1,500 men each year. Although the overall risk of those diseases is low, anal cancer in gay and bisexual men has been rising in recent years.
Worldwide, the consequences of HPV infection in both men and women are even more severe than in the United States, notes Alabama's Broker, president of the nonprofit International Papillomavirus Society.
More women in developing countries die of cervical cancer than in the United States, he says. Moreover, "we need to know how much real disease men are getting. If you look worldwide, there are about 100,000 new cases of penile cancer each year."
HPV-related cancer is also more common in people who have compromised immune systems, such as men who are HIV positive.
"This virus can cause cancers in a lot of different places," Blumberg said. "But in terms of numbers, it doesn't compare to the number of cervical cancer cases."
But even if reducing rates of cervical cancer was the singular goal of HPV vaccination, some experts suggest that herd immunity - vaccinating everybody to reduce circulation of the virus in the population - will turn out to be the most successful approach.
"If you decrease HPV infection in men, then there will be decreased transmission to women also," Blumberg says.
Merck is conducting studies of the vaccine's ability to prevent infection in boys and men. Data on those trials might become available later this year.
Studies of Gardasil show that the vaccine provokes an even stronger immune response in boys than in girls, but not whether boys are protected from HPV infection at satisfactory rates.
Some high-risk men, such as gay and bisexual men, are already requesting and receiving Gardasil vaccination from their physicians, Blumberg said.
Meanwhile, GlaxoSmithKline plans to seek Food and Drug Administration approval next month for its HPV vaccine for girls and women, Cervarix.
Lawmakers in at least 20 states have introduced bills that would require HPV vaccinations for school-age girls. But those proposals have generated controversy because some parents believe vaccinations might encourage sexual activity.
Others object to requiring vaccination for something that is not easily transmitted (unlike chickenpox or measles) and because the shots are costly, about $360 for the series.
Those arguments and others persuaded Maryland lawmakers to kill HPV legislation this year.
Shari Roan writes for the Los Angeles Times.