In 1918, WWI American soldiers unwittingly carried the deadly virus from a Kansas military camp to the European Theater of War. Flu turned up next in Poland and circled the globe, in three waves of disease that lasted until 1920. At least 20 million, possibly 40 million people died.²⁵

Death came quickly, often within days. Historians believe the virus infected half the world population of two billion people. In the mass upheavals caused by the Great War, disease spread along transportation routes from ports to cities, reaching even the remote islands and villages, including Western Samoa and Inuit settlements in Alaska. In the United States, an estimated 550,000 people died, and life expectancy dropped by 12 years.²⁶

Historians speculate that the flu helped end WWI, because troops were too sick to fight. When the numbers were counted, influenza killed more men than arms did, and may have contributed indirectly to the start of WWII. Some authorities believe that President Woodrow Wilson was so sick with flu during the negotiations to end WWI, that he failed to make the Treaty of Versailles powerful enough to help prevent WWII.²⁷