The dengue virus has an icosahedral shape, similar to the pattern on a soccer ball. Antibodies stop the virus by binding to its surface. Laguna Design/Science Source hide caption

itoggle caption Laguna Design/Science Source

This experimental Ebola vaccine, developed by the U.S. government, is just one of several undergoing small-scale, preliminary testing. University of Maryland School of Medicine/AP hide caption

itoggle caption University of Maryland School of Medicine/AP

Joseph Robert gets a flu shot at the Metropolitan Multi-Service Center in Houston during the Falls and Flu Prevention Day, sponsored by the National Council on Aging and Sanofi Pasteur, on Sept. 26, 2013. Aaron M. Sprecher/AP Images for National Council on Aging and Sanofi Pasteur hide caption

itoggle caption Aaron M. Sprecher/AP Images for National Council on Aging and Sanofi Pasteur

Boys show off their four-legged friends at a rabies vaccination drive set up by the Serengeti Health Initiative in the Bariadi District of Tanzania. Anna Czupryna/Courtesy of Serengeti Health Initiative hide caption

itoggle caption Anna Czupryna/Courtesy of Serengeti Health Initiative

Daniela Chavarriaga holds her daughter Emma as Dr. Jose Rosa-Olivares administers a measles vaccination at Miami Children's Hospital. Joe Raedle/Getty Images hide caption

itoggle caption Joe Raedle/Getty Images

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

Marqui Ducarme is aided by his wife after catching chikungunya at his home in Port-au-Prince, May 23. The virus swept through Haiti this spring, infecting more than 40,000 people. Marie Arago/Reuters/Landov hide caption

itoggle caption Marie Arago/Reuters/Landov