A vaccine gives immunity to an infectious disease caused by a particular bacterium or virus. This means the vaccine makes a person less likely to get that disease. For example, the flu vaccine makes it less likely that a person will get the flu.
Vaccines are usually made from something that is alive, or was alive.
The word "vaccine" comes from the Latin words vaccin-us (from the word vacca, meaning "cow"). In 1796, Edward Jenner used cows infected with cowpox (variolae vaccinae) to protect people against smallpox. The use of vaccines is called vaccination.
Edward Jenner created the first vaccine in the 1770s. At this time, smallpox was a deadly disease. Jenner noticed that people who had already had cowpox (a disease that is related to smallpox) usually did not get smallpox. He thought that getting cowpox protected people against smallpox.
To test this idea, Jenner gave a boy cowpox. Then he infected the boy with smallpox. The boy did not get sick because he had already had cowpox. Jenner was right: having cowpox protected people against smallpox.

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