Louis Pasteur

Pasteur proved the germ theory of disease — that microorganisms cause infection — overturning the dominant "spontaneous generation" hypothesis and transforming medicine, surgery, and public health forever. He invented pasteurization to kill pathogens in food and drink, developed the first vaccines for cholera and anthrax, and created the rabies vaccine, saving the first human patient in 1885. His work is directly responsible for modern microbiology, immunology, and the sterilization practices used in every hospital on Earth.