What is the Theory of Immunity
(1796) goes out and found someone that had cow pox, opened up one of the postules and injected a 12 year old boy with it. The boy ended up coming down with cow pox. After the boy was all healed up a couple months later he goes out and finds someone dying of small pox, opened up one of the postules and injected the same boy again but the boy doesn't get small pox!
People who had cow pox rarely ever got small pox.
Cow Pox "not fatal" Small Pox 1 in every 3 people die of small pox, high fatality rate
Louis Pasteur was aware of the cow/small pox case... he then names this