In the 1700s, smallpox was one of the most feared diseases on Earth. It killed about three out of every ten people who caught it. Survivors were often left with deep scars. Edward Jenner was a country doctor in England who made a surprising observation. Milkmaids who caught a mild disease called cowpox never seemed to get smallpox. Jenner formed a hypothesis: cowpox somehow protected people against the deadlier disease. On May 17, 1796, Jenner tested his idea. He took material from a cowpox sore on a milkmaid's hand and scratched it into the arm of an eight-year-old boy named James Phipps. The boy developed a mild fever but recovered quickly. Six weeks later, Jenner exposed James to actual smallpox. The boy stayed perfectly healthy. Jenner called his method "vaccination," from vacca, the Latin word for cow. Other doctors were skeptical at first. Some mocked Jenner with cartoons showing people growing cow parts after being vaccinated. But the evidence was clear. Vaccination worked. By 1980, smallpox became the first disease ever eliminated from the planet, thanks to the method Jenner pioneered.