Louis Pasteur was responsible for the further discovery of microorganisms as well as the pasteurisation process and vaccines
Louis Pasteur developed the idea that diseases came from microorganisms and weren't spontaneously created as commonly thought at the time. This was discovered using a swan flask which was in the shape of an S.
Using chickens, Louis used weakened microbes discovering that it can result in the immunity to a disease. This was practised on chickens with fowls cholera as well as on cows with anthrax bacilli before coming to the conclusion that it would also be able to work on human diseases as well. This was successfully first accomplished with a rabie vaccine.