Jenner wanted to test the theory scientifically.
He wanted to know what protected the milkmaids from infection.
He decided to do an experiment.
First, he infected a boy with cowpox.
Next, Dr. Jenner did a vary dangerous thing.
He infected the same boy with smallpox.
Would the cowpox virus in the boy’s body stop smallpox ?
Dr. Jenner thought so, but he did not know for sure.