Using genes resurrected from the 1918 Spanish influenza virus, the virus responsible for a pandemic that killed an estimated 20 million people, scientists have gathered important new clues to why the virus spread quickly and killed efficiently. Adding the genes to a comparatively benign strain of influenza (pictured), shows that a minor genetic change can turn a mild form of the virus into a highly virulent strain. (Photo courtesy of Yoshihiro Kawaoka, University of Wisconsin-Madison)