How vaccines work: how a shot trains your immune system
The first vaccine: how a cow disease stopped smallpox
In 1796, an English country doctor put a cow disease into a boy's arm. Weeks later, he tried to give the same boy smallpox, and nothing happened. This strange test gave us the word "vaccine".
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Long before measles shots, a worse disease went around the world: smallpox. In Europe in the 1700s, it killed about 400,000 people every year. Many of those who lived had deep scars.
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In the English countryside, people told a story. Dairymaids caught cowpox, a mild disease, from the cows they milked. And afterwards, they did not seem to catch smallpox.
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Edward Jenner, a country doctor, decided to test the story. He took a little fluid from the cowpox sores on the hand of a dairymaid, Sarah Nelmes. He put it into small cuts on the arm of James Phipps, a boy of eight.
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James had a light fever and felt unwell for a day or two. About ten days later, he was fine again.
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In July, Jenner did something no doctor would ever do today. He put fluid from a smallpox sore into James's arm.
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No disease came. The mild cowpox had left James's body a memory, and that memory also worked against smallpox.
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Jenner named the method after the cow. In Latin, a cow is "vacca". So the shot became a vaccine, and giving it became vaccination. A vaccine is a safe first meeting: you get the memory without the illness.
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Jenner never knew why it worked. Today we know that cowpox and smallpox are close cousins, and they look alike. So the body must know a germ by the way it looks.
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Go further
Two more facts about smallpox and cowpox.