> In 1796 Jenner noticed that milkmaids who had caught the cowpox virus did not normally then catch smallpox. Cowpox was very similar to smallpox but less contagious.

> He collected pus from the cowpox blisters on a milkmaid's hands and purposely infected a small boy.

> The boy was taken ill for a short while, but was then resistant to any subsequent infections of the cowpox and smallpox viruses.

> He tested this by infecting the boy with smallpox and no illness occurred.

Gosh, medical testing has come a long way since then. It's a little hard to believe that the discovery of one of the most important vaccines in human history played out this way. Someone doing something similar now would likely be executed or thrown in prison for life, and rightfully so. But, still crazy to think about.