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

A high-resolution image of the molecular carrier that moves the botulinum toxin from the intestine into the bloodstream. The carrier (silver) creates gaps in the gut lining by grabbing the rope-like molecules (red ribbons) that tether one intestinal cell to the next. Rongsheng Jin, UC Irvine, and Min Dong, Harvard Medical School hide caption

itoggle caption Rongsheng Jin, UC Irvine, and Min Dong, Harvard Medical School

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

The anthrax vaccine has been given to more than 1 million adults in the military. But no one knows how well it would work in children. Randy Davey/Reuters/Landov hide caption

itoggle caption Randy Davey/Reuters/Landov