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

Public school student Julio Valenzuela, 11, grimaces as he gets a vaccination before the start of the school in Lynnwood, Calif., on Aug. 27. Vaccines are required for school attendance. ROBYN BECK/AFP/Getty Images hide caption

itoggle caption ROBYN BECK/AFP/Getty Images

Each dot represents one case of mumps between late June 2009 and late June 2010. The New England Journal of Medicine hide caption

itoggle caption The New England Journal of Medicine

Two boys study together at a Chicago yeshiva in 2009. Public health officials say this type of close physical contact caused a mumps outbreak to spread throughout several orthodox Jewish communities in and around New York City. M. Spencer Green/AP hide caption

itoggle caption M. Spencer Green/AP