How does a vaccine train your body to fight a germ it has never met?

A vaccine works like a wanted poster for your immune system. It contains something that resembles a dangerous microbe, often a weakened or killed version of it, one of its toxins, or just one of its surface proteins, but without the power to make you seriously ill. Your immune system treats it as a threat, destroys it, and remembers it. If the real germ ever shows up, your body is ready to neutralize it before it gets into your cells and to wipe out infected cells before the germ can multiply. The training takes time: full adaptive immunity can take one to two weeks to develop.

The idea began with cows. In 1796, the English doctor Edward Jenner took pus from cowpox blisters on the hands of a milkmaid, Sarah Nelmes, who had caught the mild disease from a cow called Blossom. He scratched it into the arm of James Phipps, his gardener's eight-year-old son. Weeks later he exposed the boy to smallpox, a disease that killed around one in ten people at the time. James didn't get sick.

Jenner called cowpox Variolae vaccinae, 'pustules of the cow', which gave us the word vaccine. Less than two centuries later, in 1980, the World Health Organization declared smallpox eradicated, the only human disease ever wiped out.