The CDC's director, Tom Frieden, testified before a congressional subcommittee Wednesday regarding a recent anthrax incident and lab safety improvements he is instituting. AFP/Getty Images hide caption

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Particles of H5N1 virus — a particularly dangerous type of bird flu that can infect people — attack lung cells. Chris Bjornberg/Science Source hide caption

itoggle caption Chris Bjornberg/Science Source

U.S. Marine Sgt. Robert Scoggin gets a vaccination against smallpox in 2003 at Camp Pendleton in California — one of the final steps before deployment overseas. David McNew/Getty Images hide caption

itoggle caption David McNew/Getty Images