Before Louis Pasteur, the medical establishment believed that diseases were caused by bad air, moral failings, or the alignment of celestial bodies. The idea that tiny organisms invisible to the naked eye could sicken and kill human beings was considered absurd by many of the most educated physicians in Europe. Pasteur proved them wrong. They did not thank him for it. He was a chemist, not a doctor. This made the doctors even angrier.

He Saved the Wine Industry Before He Saved Human Lives

Pasteur's first major breakthrough had nothing to do with medicine. In 1856, a French industrialist asked him why his beet juice kept turning sour instead of fermenting into alcohol. Pasteur examined the vats under a microscope and discovered that contaminating microorganisms were disrupting the fermentation process. He developed a heating technique that killed the unwanted microbes without destroying the product. This technique, which became known as pasteurization, saved the French wine and beer industries before it saved a single human life. Historians of science at the Institut Pasteur in Paris have documented that Pasteur's initial fame came not from curing disease but from making sure French people could drink their wine without it turning to vinegar. The insight that microorganisms caused fermentation led directly to the insight that microorganisms caused disease. This was germ theory, and it was one of the most important scientific ideas in human history. It was also ferociously resisted by physicians who had built their entire careers on the miasma theory of disease, which held that sickness came from foul smells and rotting matter.

He Made a Vaccine for Rabies by Testing It on a Nine-Year-Old Boy

In 1885, a woman brought her nine-year-old son, Joseph Meister, to Pasteur's laboratory. The boy had been bitten fourteen times by a rabid dog. Without treatment, rabies was one hundred percent fatal. There was no treatment. Pasteur had been developing a rabies vaccine using dried spinal cords from infected rabbits. He had tested it on dogs. He had never tested it on a human being. He was not a licensed physician. Administering the vaccine was, strictly speaking, illegal. He did it anyway. Medical historians at Johns Hopkins University have described the Meister case as one of the most consequential gambles in the history of medicine. Pasteur administered thirteen injections over ten days, each containing a progressively stronger strain of the virus. The boy survived. The vaccine worked. Pasteur was hailed as a hero. He was also criticized for recklessness. Both reactions were justified. He had risked a child's life on an unproven treatment. He had also saved it. The line between those two things is the line that medical ethics has been trying to draw ever since. He founded the Institut Pasteur in 1887. He suffered multiple strokes in his later years and died in 1895, surrounded by his family. His last words, according to his grandson, were about wanting to do more. He had disproved spontaneous generation, invented pasteurization, developed vaccines for anthrax and rabies, and established the germ theory of disease. He still thought it was not enough. The invisible things he discovered are still killing people. The methods he developed are still saving them.