This did not really happen. Cows' heads did not emerge from the bodies of people newly inoculated against smallpox. But fear of the vaccine was so widespread that it prompted British satirist James Gillray to create this spoof in 1802. H. Humphrey/Henry Barton Jacobs Collection, Institute of the History of Medicine, JHU hide caption

itoggle caption H. Humphrey/Henry Barton Jacobs Collection, Institute of the History of Medicine, JHU

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