In 1796, a country doctor in England noticed that milkmaids who'd caught a mild disease called cowpox never got smallpox, one of history's deadliest plagues. He infected an 8-year-old boy with cowpox to test the theory. It was the beginning of the end for a disease that had killed 500 million people.

Smallpox was unimaginably horrific. It killed roughly 30% of everyone it infected and left survivors blind or heavily scarred. It killed three Japanese emperors, Louis XV of France, and Queen Mary II of England. It wiped out up to 90% of some Native American populations after European contact. Throughout the 18th century, it was the leading cause of death in Europe.

The observation Edward Jenner built on wasn't new. Dairymaids had long noted their immunity, and practices of deliberate smallpox infection — 'variolation' — existed in China, India, and the Ottoman Empire for centuries. But variolation used actual smallpox virus, and 1-2% of people died from the treatment itself.

Jenner's insight was using cowpox instead. Cowpox caused mild illness in humans but somehow protected against smallpox. On May 14, 1796, he extracted pus from a blister on milkmaid Sarah Nelmes and inserted it into cuts on the arm of James Phipps, the son of his gardener. Six weeks later, he exposed the boy to actual smallpox. Phipps didn't get sick.

The Latin word for cow is vacca. Jenner called his procedure vaccination.

The resistance was fierce. Cartoonists depicted vaccinated patients growing horns and cow features. Clergymen called it unnatural. But the evidence was undeniable: in populations that vaccinated, smallpox deaths plummeted. The Ottomans mandated vaccination in 1840. England made it compulsory for infants in 1853.

Then, in 1967, the World Health Organization launched an unprecedented campaign to eradicate smallpox entirely. Teams hunted every remaining case on Earth, vaccinating everyone nearby. The last naturally occurring case was in Somalia in 1977. In 1980, smallpox was declared eradicated — the first and still the only human disease ever eliminated.

A humble observation about dairymaids, tested on a gardener's son, ended a plague that had stalked humanity for at least 3,000 years.