The research of Louis Pasteur focused on problems of industry, such as demonstrating that organisms like bacteria were responsible for souring wine, beer, and milk.
Pasteur found that boiling and cooling liquid, a process that came to be known as pasteurization, removed bacteria. He saved the silk industry by identifying parasitic infections in the eggs of silkworms.
Such work convinced him of germ theory: that germs attack the body from outside, not from within. He proved that bacteria are introduced from the environment, not spontaneously created.
Many had thought a tiny organism couldn’t harm a large one i.e. a human body. He extended the theory to disease, specifically causes and means of prevention from anthrax, cholera, TB and smallpox. He is also known for the development of the rabies vaccine. He became the Director of the Pasteur Institute for the Treatment of Diseases.