In 1918, an influenza epidemic killed an estimated 20 million people worldwide and 548,000 in the U.S. alone. It’s devastating effect contributed to the German decision to seek an armistice.
Richert, Lucas Paul. "Influenza epidemic". The Literary Encyclopedia. First published 01 November 2008
[http://www.litencyc.com/php/stopics.php?rec=true&UID=5574, accessed 20 December 2014.]