When American colonists launched their revolution against Britain, they quickly encountered a second but invisible enemy that threatened to wipe out the new Continental Army: highly contagious smallpox.

But luckily for the young nation, the army’s commander was familiar with this formidable foe. George Washington’s embrace of science-based medical treatments—despite stiff opposition from the Continental Congress—prevented a potentially disastrous defeat, and made him the country’s first public health advocate.

A hard lesson

Washington’s wisdom came from personal experience with the horrors of an epidemic. “Was strongly attacked by the small Pox,” Washington wrote as a teenager in 1751, while visiting the Caribbean island of Barbados. At the time, the disease caused by the variola virus killed as many as one in two victims. Washington was lucky. After nearly a month of chills, fever, and painful pustules, he emerged with the pockmarked face typical of survivors—but alive, and with immunity to the illness.

Washington’s encounter with the virus proved fortunate for the new nation. In 1775, smallpox arrived in Boston, carried by troops sent from Britain, Canada, and Germany to stamp out the growing rebellion. Many of these soldiers had been exposed and were therefore immune, but the vast majority of American colonists were not.

In the aftermath of the battles of Lexington and Concord, Washington’s Continental Army had set up camp across the Charles River from the stricken city. To the dismay of many patriots seeking refuge from the British, the general prohibited anyone from Boston from entering the military zone. “Every precaution must be used to prevent its spreading,” he sternly warned one of his subordinates about the virus. To John Hancock, the president of the Continental Congress, Washington vowed to “continue the utmost Vigilance against this most dangerous enemy.”

By immediately isolating anyone suspected of infection and limiting outside contact, Washington “prevented a disastrous epidemic among the Continental troops,” historian Ann Becker says. In March 1776, when the British withdrew from Boston, Washington even specified that only soldiers who had suffered from smallpox be allowed into the city and its surroundings.