I am going to give you a dose of cowpox first, some pus that's come from a milk maid's sores who had cowpox.
Owww, this hurts a lot!
And now that you've recovered from your cowpox, I'm going to give you a does of smallpox.
Edward Jenner was an English physician who found out a way to stop young children from getting smallpox through the use of vaccinations.
He found that milk maid's didn't get smallpox as they were near cows a lot of the time. He took out his first experiment on a young boy called James Phipps. He put pus from sores of a woman who had cowpox into the young boy. He got cowpox like the milk maid. After he had recovered, Jenner gave him a dose of smallpox but the boy didn't catch it.
After James Phipps had recovered from his cowpox, he got a dose of smallpox. Amazingly, James did not go down with smallpox as he had already had cowpox, therefor protecting him from catching smallpox.