In 1918 the Spanish flu swept the globe, killing at least 50 million people worldwide. That's over three times as many deaths as in World War I.
In total, 650,000 Americans died from the Spanish flu, named after disease's early presence in Spain.
Scientists have now identified the Spanish flu as H1N1, an avian strain of influenza just like the currently feared H5N1. In 2006, the White House released a plan for a world influenza outbreak that suggested that up to two million Americans could die if a flu pandemic occurred.
If you think that number is far fetched, click through the slides and see how the Spanish flu brought the world to its knees not so long ago.