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

Red blood cells infected with the Plasmodium falciparum parasite. Plasmodium is the parasite that triggers malaria in people. Gary D. Gaugler/Science Source hide caption

itoggle caption Gary D. Gaugler/Science Source

A doctor gives a polio vaccine to a child at a health clinic in Baghdad last week. The CIA says it banned the use of vaccine programs as cover for spying last year — a practice health officials said had wide repercussions. Ahmad Al-Rubaye/AFP/Getty Images hide caption

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False-color transmission electron micrograph of a field of whooping cough bacteria, Bordetella pertussis. A. Barry Dowsett/Science Source hide caption

itoggle caption A. Barry Dowsett/Science Source

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

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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 school nurse prepares a vaccine against whooping cough before giving it to students at Mark Twain Middle School in Los Angeles. Kevork Djansezian/Getty Images hide caption

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