To test this saying, Jenner gave his son Edward Jr. the cowpox. A few days later, he tried to infect this same son with smallpox and couldn't. Jenner had unknowingly invented the first vaccine against the virus smallpox. Almost a hundred years after Jenner's first vaccine experiments, another scientist, this time in France, furthered the battle against the virus. Louis Pasteur was a well-known chemist who had been studying the effects of rabies.
People usually catch rabies from being bitten by wild animals.
Rabies is a virus that affects the brain and often lead to death in Pasteur's time. He noticed that as rabies was passed from creature to creature it seemed to get less powerful. So if a bat bit a dog, who bit a cat who then bit a human, the bat would have the worst case of rabies and the human would have a least serious case. Pasteur thought that if he could take a sample of blood from someone who had a very weak case of rabies, and somehow transfer it into the body of someone with serious rabies, they could be cured. Pasteur took a small boy who had been bitten by a rabid dog and tested this theory by injecting him with a weakened rabies form.