Louis Pasteur

Louis Pasteur was one of the great French scientists, famous for his work with curing rabies and developing the idea of vaccination. He also championed the radical idea that dirty hands, clothes and instruments in hospitals contributed to the spread of disease, and basically demonstrated the germ theory of disease to be correct, although not in every case.

While a student at École Normale Supérieure, Pasteur studied crystallography, which led him to the puzzle of why tartaric acid (found in grapes and wine) bent light passed through it, but para-tartaric did not. From examining the crystals of the two chemicals' salts, Pasteur found that tartaric acid crystals formed only right-handed crystals but that the para-tartaric acid crystals formed both right- and left-handed crystals. From this, he deduced that the uniform handedness of tartaric acid was the reason that it alone could bend light and that the handedness was also reflected in the chemicals' structures. His research on wine chemicals led to his work on microbes and disease. At the time yeast was believed to be a chemical that acted like a catalyst in wine fermentation, but Pasteur was able to demonstrate that yeast was like a small plant (what is now called a fungus), and that yeast was dependent on nutrition (sugar) to reproduce. Pasteur was able to demonstrate that contaminating microbes were at fault for making bad wine, the discover of which saved the French wine industry. Pasteur was able to similarly show that microbes were at fault in bad beer and milk that caused sickness, and heat treatment (pasteurization) would solve the problems of these industries. In 1857, having proven the germ theory of disease, Pasteur summarized his work in a paper that marks the beginning of field of scientific microbiology. Pasteur continued his research in microbiology and human health, by improving on Edward Jenner vaccination method by developing a generalized method using weakened microbes and injection methods for cholera, anthrax, rabies, and silkworm disease.

All in all, he was about as practical a scientist as one could possibly be, a brilliant man to whom millions of people owe their lives and one who it can truly be said made a difference in the lot of nearly all people alive today.

Pasteurization
Pasteurisation, the process of heating/cooling food to kill germs, was also an idea that began with Pasteur (hence its name). In much of the food industry, particularly the dairy industry, it is considered one of the most important front-line sanitation procedures.

It is not so popular with certain food faddists, who believe that pasteurization destroys the nutrition of milk, honey, and juice. Such is it ever with those with short memories of harder times.

Pasteurization does have the drawback that, as it's a heat treatment, it destroys those vitamins that break down when heated. For that reason, pasteurized milk is usually sold with Vitamin D and vitamin A enrichment. It does not, however, destroy macronutrients — proteins, fats, and carbohydrates. If a food item is heated so hot that its macronutrients are damaged, we don't call it pasteurization, we call it burning.

Cranks
Pasteur is beloved of creationists because he disproved spontaneous generation. They completely ignore, as is their forte, the fact that their idea of spontaneous generation and the idea that Pasteur disproved are vastly different things. He disproved the idea that bacteria formed in a broth of their own accord, using a flask which bears his name. Creationists think™ that he disproved the idea of a non-supernatural abiogenesis event.

That said, there are still those who consider vaccination and pasteurisation to be the work of the devil (or at least a big agribusiness conspiracy), although these processes have saved countless lives over the years. Vaccine denialism and germ theory denialism still flourish today in spite of mountains of scientific evidence as to their efficacy.