Louis Pasteur was a French biologist and chemist whose discoveries in the causes and prevention of diseases laid the foundation for modern hygiene and public health. He is best known for proving the 'germ theory of disease,' which argued that microorganisms are the cause of many illnesses, debunking the then-prevalent idea of spontaneous generation. Pasteur developed the process of 'pasteurization,' a heat treatment that kills harmful bacteria in beverages like milk and wine. He also created the first vaccines for rabies and anthrax. His work shifted medicine from a field of observation to one of experimental science, drastically reducing mortality rates worldwide. By demonstrating that invisible 'germs' could be controlled, he paved the way for the development of antiseptics and modern immunology.