When George Washington took command of the Continental Army in 1775, America was fighting a war on two fronts: one for independence from the British, and a second for survival against smallpox. Because Washington knew the ravages of the disease firsthand, he understood that the smallpox virus, then an invisible enemy, could cripple his army and end the war before it began.

That’s why Washington eventually made the bold decision to inoculate all American troops who had never been sickened with smallpox at a time when inoculation was a crude and often deadly process. His gamble paid off. The measure staved off smallpox long enough to win a years-long fight with the British. In the process, Washington pulled off the first massive, state-funded immunization campaign in American history.

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“Washington knew what smallpox was like and he knew how it could incapacitate his Army,” says Elizabeth Fenn, a professor of early American history of the University of Colorado Boulder and author of Pox Americana: The Great Smallpox Epidemic of 1775-82. 

Washington also knew that his American-born soldiers were far more susceptible to the disease than the European enemy. That’s because smallpox was endemic in England, meaning that a high percentage of British troops had already contracted the disease as children and now carried lifelong immunity.