Bruno Mbango Enyaka gets his flu shot at a community health center in Portland, Maine, on Jan. 7. Gabe Souza/Press Herald via Getty Images hide caption

itoggle caption Gabe Souza/Press Herald via Getty Images

The human papilloma virus causes most — but not all — cases of cancer of the cervix. James Cavallini/ScienceSource hide caption

itoggle caption James Cavallini/ScienceSource

One teaspoon of pure caffeine powder delivers about the same jolt as 25 cups of coffee. The Center for Science in the Public Interest hide caption

itoggle caption The Center for Science in the Public Interest

Stringy particles of Ebola virus (blue) bud from a chronically infected cell (yellow-green) in this colorized, scanning electron micrograph. NIAID/Science Source hide caption

itoggle caption NIAID/Science Source

Several countries, including Australia, Japan and Great Britain, already encourage blood donations from some gay men. Kevin Curtis/Getty Images/Science Photo Library hide caption

itoggle caption Kevin Curtis/Getty Images/Science Photo Library