In 1918 and 1919, the Spanish flu killed an estimated 50 million people worldwide. Today, health experts worry that if the virulent avian flu were to mutate into a strain that humans could easily contract and spread, the world could face a similarly devastating pandemic.

In an effort to develop new flu vaccines more quickly and at lower cost, researchers funded by major bodies like the National Institutes of Health and the U.S. Centers for Disease Control and Prevention are working on vaccines for the 1918 flu virus itself. They hope to learn what made it so contagious and deadly.