Avez, 2, is held by his mother, as he receives the polio vaccine in the Khyber Tribal Region in northwest Pakistan. Daniel Berehulak/Getty Images hide caption

itoggle caption Daniel Berehulak/Getty Images

A student gets vaccinated against pertussis at a Los Angeles middle school in 2012. The state required that students be immunized to halt an epidemic of whooping cough. Kevork Djansezian/Getty Images hide caption

itoggle caption Kevork Djansezian/Getty Images

The Eagle Mountain Church in Newark, Texas, was linked to at least 21 cases of measles this year, mostly in children. LM Otero/AP hide caption

itoggle caption LM Otero/AP

A red blood cell infected with malaria parasites (blue) sits next to normal cells (red). NIAID/Flickr.com hide caption

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Ads often tout dietary supplements and vitamins as "natural" remedies. But studies show megadoses of some vitamins can actually boost the risk of heart disease and cancer, warns Dr. Paul Offit. iStockphoto.com hide caption

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Luke Tanner, 7, gets vaccinated for measles at a clinic near Swansea, Wales, in April. Wales is at the center of a measles outbreak that has been linked to one death. Geoff Caddick/AFP/Getty Images hide caption

itoggle caption Geoff Caddick/AFP/Getty Images

A new study from the Centers for Disease Control and Prevention finds no link between the number of vaccinations a young child receives and the risk of developing autism spectrum disorders. Jeff J. Mitchell/Getty Images hide caption

itoggle caption Jeff J. Mitchell/Getty Images

A simple switch of ingredients made a big difference in how mice responded to experimental cancer vaccines. Andrei Tchernov/iStockphoto.com hide caption

itoggle caption Andrei Tchernov/iStockphoto.com

Nurse Christel Petersen inoculates a child in the South African Tuberculosis Vaccine Initiative study in 2011. Rodger Bosch/AFP/Getty Images hide caption

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The fall armyworm, a corn pest, is now also a vaccine factory. Wikimedia Commons hide caption

itoggle caption Wikimedia Commons

When flu viruses (in red) accumulate an escape protein too quickly, they exit the cell nucleus (in blue) before they've made enough viral copies to spread the infection. Benjamin tenOever hide caption

itoggle caption Benjamin tenOever

Some parents have worried that kids get too many vaccinations too quickly. A review of all the available research suggests those concerns are misplaced. Dmitry Naumov/iStockphoto.com hide caption

itoggle caption Dmitry Naumov/iStockphoto.com