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.
Citation:
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 18 May 2013.]