The immune system remembers. Show it a harmless version of a threat and it builds defences, so that when the real pathogen arrives it is met and beaten before it can take hold. In 1796 Edward Jenner acted on a country observation — milkmaids who caught mild cowpox never got smallpox — and deliberately gave a boy cowpox, then exposed him to smallpox. The boy stayed well. That was the first vaccine. Refined and spread over two centuries, it ended with smallpox — a disease that killed perhaps 300 million people in the twentieth century alone — being declared eradicated in 1980, the only human disease ever erased.
Vaccination is among the few interventions that have saved lives by the hundreds of millions, and the only one that has driven a human disease to extinction. Every modern vaccine descends from Jenner’s insight.
This is the identical fact set, re-told at a different altitude. Switch any time — the reader keeps your place in the idea, not the prose.