The New World of the Western Hemisphere was devastated by the 1775–1782 North American smallpox epidemic.
This epidemic occurred during the years of the American Revolutionary War. During this time, there was no medical technology widely available to protect soldiers from outbreaks in crowded and unhygienic troop camps. Thus, this virus posed a major threat to the success of the Continental Army, led by George Washington.
https://en.wikipedia.org/wiki/1775%E2%80%931782_North_American_smallpox_epidemic