In this November 1918 photo made available by the Library of Congress, a nurse takes the pulse of a patient in the influenza ward of the Walter Reed hospital in Washington. Historians think the pandemic started in Kansas in early 1918, and by winter 1919 the virus had infected a third of the global population and killed at least 50 million people, including 675,000 Americans. Some estimates put the toll as high as 100 million. By comparison, the AIDS virus has claimed 35 million lives over four decades. (Harris & Ewing/Library of Congress via AP)
