What are Louis Pasteur contribution to the study of disease?

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[2] 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.