In the eighteenth century smallpox (variola) was a terror that killed hundreds of thousands each year and left survivors blind and deeply scarred. It was known that those who survived the disease never caught it again; so the practice of deliberately infecting a healthy person with a mild case (variolation), used in the Ottoman world and Asia, had been carried to Europe too — but this method sometimes triggered the real disease and killed.

Edward Jenner (1749–1823), a physician in the English countryside, took seriously a rumour circulating among country folk: milkmaids who caught the mild "cowpox" from cows then never fell ill with deadly smallpox. Jenner decided to test whether this conferred immunity.

In 1796 he took some of the pus from a cowpox blister on the hand of a milkmaid and inoculated an eight-year-old boy (James Phipps). The boy had a mild case of cowpox and recovered. Then — within the ethical limits of the time — he exposed the boy to the real smallpox virus; the boy did not fall ill. The immunity had worked. Jenner named this method "vaccination," from the Latin *vacca*, meaning cow.

The discovery rested entirely on observation, with no knowledge of microbes or the immune system; Jenner could not explain why it worked. But the method spread quickly across the world. A century later Pasteur extended the same principle to rabies and anthrax; in the twentieth century vaccines pushed back dozens of diseases from measles to polio. In 1980 the World Health Organization officially declared smallpox "eradicated" — the first and only time humanity has wholly erased a disease. It all began with a country doctor turning a rumour into an experiment.