Cowpox
Some of his patients had gotten cowpox, and recovered from it. He observed that those patients did not get the disease again, they seemed to be immune against it. What was more, they also seemed to be immune against smallpox. Smallpox was a deadly disease then, that killed most of the people it infected. So he used the fluid he got from cowpox lesions, and scratched it into healthy people. That way, he could make those people immune against smallpox.