This is progress. In the last 10 years, the cases of human papillomavirus have dropped by 50 percent in teenage girls. The vaccine debuted in 2006 and combats the HPV virus that exists in 70 percent of cervical-cancer cases. The results surprised experts since vaccinations rates are still pretty low. Another explanation for the decline is herd immunity—in which those vaccinated decrease the chance of the virus proliferating in a society. HPV is the most common sexually transmitted infection in the United States—at least for now.