Five years ago the Centers for Disease Control and Prevention controversially recommended that girls be vaccinated to prevent infection with human papilloma virus (HPV). Now the agency is extending the recommendation to boys ages 11 to 13, with a onetime “catch-up” injection suggested between ages 13 and 21. The vaccine protects against four common strains of the virus, considered the most common sexually transmitted disease in the United States. HPV is responsible for triggering genital warts as well as cervical, vaginal, vulvar, anal, and oropharyngeal cancers. “We estimate 11,000 HPV-associated cervical cancers and 4,000 HPV-associated anal cancers each year for men and women,” says epidemiologist Lauri Markowitz, a member of the CDC’s Advisory Committee on Immunization Practices. The vaccine has not yet been widely adopted, in part because of resistance to vaccinating preteens against a disease spread by sexual activity. But Markowitz notes that if boys are vaccinated, their female partners will be protected too.