This work led Pasteur into a more thorough study of bacteria, enabling him to prove that these microscopic organisms occurred naturally in the environment and did not simply appear spontaneously, as was then generally believed.

As the director of scientific studies at the Ecole Normale in Paris, Pasteur pursued his germ theory, which posited that germs attack the body from the outside. Proved right again, his work led to vaccinations being developed for many germ-borne diseases, including anthrax, tuberculosis, cholera and smallpox. It also led to further work on rabies, which was much more prevalent in Pasteur's time than it is today.

He developed his rabies vaccine by growing the virus in rabbits, then drying the affected nerve tissue to weaken the virus.

On July 6, 1885, the vaccine was administered to Joseph Meister, a 9-year-old boy who had been attacked by a rabid dog. The boy survived and avoided contracting rabies, which would have almost certainly proved fatal.

Good thing it worked: Pasteur was not a licensed physician and could have been prosecuted had the vaccine failed. The legalities were forgotten and Pasteur instead became a national hero.

According to wikipedia