Dying of smallpox was not a pleasant way to go, with a slow and painful death almost guaranteed. On the other hand, catching cowpox was only a minor inconvenience and, better yet, it prevented you from catching smallpox. English doctor Edward Jenner noticed this link and, after experimenting on some of the local diary workers, published his results in 1798. He invented a vaccine that later became mandatory, though the majority of the people rejected this idea before his research was finally recognized.
