Step 1: The correct answer is Louis Pasteur.
Step 2: Louis Pasteur was a French scientist who developed the first successful rabies vaccine in 1885.
Step 3: He tested it on a boy who had been bitten by a rabid dog, and the treatment saved the boy's life, marking a major milestone in medicine.
Step 4: Pasteur is also known for germ theory and for developing the process of pasteurisation used to make milk and other foods safer to consume.