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The biggest discovery in medicine of all time, the one that saved the most lives, was immunisation. The idea is simple: you put a very small amount of a disease into a body – but not enough to make the person sick. The body’s immune system makes antibodies to fight the disease. If the person catches the disease later, his body is ready and fights it so that he does not become ill and die. This is the story of the history of immunisation (800 words).