Rhett Krawitt, 6, outside his school in Tiburon, Calif. Seven percent of the children in his school are not vaccinated. Courtesy of Carl Krawitt hide caption

itoggle caption Courtesy of Carl Krawitt

Helen Down holds her 14-month-old daughter, Amelia, for an MMR shot in Swansea, Wales, April 2013. The vaccination was in response to a measles outbreak. Geoff Caddick/AFP/Getty Images hide caption

itoggle caption Geoff Caddick/AFP/Getty Images

Amish show up at a makeshift clinic to get vaccinated against the measles. There's been an outbreak of measles among the Amish in central Ohio. Sarah Jane Tribble/Sarah Jane Tribble hide caption

itoggle caption Sarah Jane Tribble/Sarah Jane Tribble

There today, here tomorrow: A mother holds her child for a measles vaccination in Manila, Philippines, in January. Travelers are bringing measles from the Philippines to the United States. Noel Celis/AFP/Getty Images hide caption

itoggle caption Noel Celis/AFP/Getty Images

This one's virus-free: Matthew Followill, Nathan Followill and Caleb Followill of Kings of Leon performed in Los Angeles in December. Kevin Winter/Getty Images for Radio.com hide caption

itoggle caption Kevin Winter/Getty Images for Radio.com

A boy waits to get vaccinated at an anti-polio campaign in Moradabad, India. Michaeleen Doucleff/NPR hide caption

itoggle caption Michaeleen Doucleff/NPR