Pasteur demonstrated that fermentation is caused by the growth of micro-organisms, and that the emergent growth of bacteria in nutrient broths is not due to spontaneous generation but rather to biogenesis. While Pasteur was not the first to propose germ theory, he developed it and conducted experiments that clearly indicated its correctness and managed to convince most of Europe it was true. Today he is often regarded as the father of germ theory and bacteriology, together with Robert Koch.
Pasteur's later work on diseases included work on chicken cholera. During this work, a culture of the responsible bacteria had spoiled and failed to induce the disease in some chickens he was infecting with the disease. Upon reusing these healthy chickens, Pasteur discovered that he could not infect them, even with fresh bacteria; the weakened bacteria had caused the chickens to become immune to the disease, even though they had caused only mild symptoms.
The notion of a weak form of a disease causing immunity to the virulent version was not new; this had been known for a long time for smallpox. Inoculation with smallpox was known to result in far less scarring, and greatly reduced mortality, in comparison with the naturally acquired disease. Edward Jenner had also discovered vaccination, using cowpox to give cross-immunity to smallpox (in 1796), and by Pasteur's time this had generally replaced the use of actual smallpox material in inoculation. The difference between smallpox vaccination and antharax or chicken cholera vaccination was that the weakened form of the latter two disease organisms had been generated artificially, and so a naturally weak form of the disease organism did not need to be found.
This discovery revolutionized work in infectious diseases, and Pasteur gave these artificially weakened diseases the generic name of vaccines, in honour of Jenner's discovery. Pasteur produced the first vaccine for rabies by growing the virus in rabbits, and then weakening it by drying the affected nerve tissue.
His contributions are quite nemerous, actually. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerpal fever,and he created the first vaccine for rabies and anthrax. His experiments supported the germ theory of disease. In case you are wondering, the germ theory is, perhaps obviously, the theory that microorganisms, or germs, are the cause of many diseases. As you probably know, this is now a cornerstone in science and medicine. Without this theory, we wouldn't have had antibiotics. Also, I'm not sure if this is important, you may already know he is the inventor of pasteurization (eponymous). Hope this helps! :)