Who was Louis Pasteur?
Germs, pasteurization, vaccines: how a French chemist proved the invisible was alive — and roughly doubled human life expectancy.
Last updated: October 2026
For all of history, the greatest killer of human beings was invisible — not war, not famine, but the unseen world of microbes. Louis Pasteur dragged it into the light. Born in 1822 in Dole, in eastern France, the son of a tanner who had served as a sergeant in Napoleon's armies, Pasteur began as a chemist, became a biologist by accident, and ended as the founder of microbiology — the man who proved that diseases are caused by living germs, that food can be made safe by heating it, and that the body can be taught to defend itself.
Pasteur's career reads like a chain of rescues. He saved the beer and wine industries of France from spoilage. He saved the silk industry from a plague that was killing silkworms. He saved cattle from anthrax and chickens from cholera. And in 1885, he saved a boy named Joseph Meister, bitten by a rabid dog, with the first rabies vaccine — the moment when vaccination, until then a single procedure against smallpox, became a general weapon against infectious disease. Pasteur did not work alone, and he was often wrong in detail, but no single life did more to move medicine from guesswork to science.
Pasteur trained as a chemist, not a doctor. His first scientific work, as a young man in the 1840s, concerned the crystals of tartaric acid — the deposits left in wine barrels. He discovered that two forms of the acid, identical in chemical composition, rotated polarized light in opposite directions, and that the difference came from the shape of their crystals: one form was the mirror image of the other. The discovery of molecular asymmetry made his name in chemistry, but it also planted the seed of his life's work: the difference between the living and the non-living is written in the geometry of molecules.
Pasteur's next problem came from industry. In Lille, in the 1850s, a beetroot alcohol manufacturer asked him why his fermentation vats kept going sour. Pasteur's answer, after months at the microscope, was revolutionary: fermentation is not a chemical process. It is a biological one — the work of living yeast. And souring was the work of other, unwanted microbes invading the vat. The germ theory of fermentation was the first crack in the old idea that decay and disease arise spontaneously from dead matter. Pasteur would spend the rest of his life widening that crack.
The doctrine Pasteur attacked was spontaneous generation: the ancient belief that life arises by itself from non-living matter — maggots from meat, microbes from broth. In the 1860s, Pasteur staged one of the most elegant experiments in the history of science. He boiled broth in glass flasks with long, S-shaped necks — the swan-neck flask. Air could enter freely, but dust, and the microbes riding on it, settled in the curve of the neck and never reached the broth. The broth stayed clear indefinitely. When Pasteur broke off the neck, dust fell in, microbes grew, and the broth clouded within days.
The experiment was decisive, and Pasteur defended it in public debate with the naturalist Félix Pouchet, who believed in spontaneous generation. The French Academy of Sciences awarded Pasteur the prize in 1862. The lesson went far beyond broth: if microbes do not appear by themselves, then they come from somewhere — from the air, from water, from contact. And if they come from somewhere, they can be stopped. The whole of modern hygiene, from the sterile operating theatre to the sealed food jar, follows from the swan-neck flask.
Pasteur's work on fermentation led directly to his most famous practical invention. Wine and beer, France's great industries, were constantly spoiled by contaminating microbes that turned wine to vinegar and beer sour. Pasteur showed that heating the liquid briefly — to around 50 to 60 degrees Celsius — killed the spoilage microbes without ruining the drink. The process, named pasteurization after him, saved the wine trade, and it would later be applied to milk, where it became one of the great public health measures in history, cutting the transmission of tuberculosis, typhoid, and other diseases through milk.
Pasteurization embodies Pasteur's method: find the microbe, understand its habits, and defeat it with the smallest intervention that works. He never sought to sterilize the world. He sought to control the conditions under which microbes live and die. The same method would carry him from the brewery to the hospital, from the vineyard to the farm.
In the 1860s, the silk industry of southern France was being destroyed by pébrine, a disease killing silkworms. Pasteur, who knew nothing about silkworms, spent five years on the problem, and discovered that pébrine was caused by a microscopic parasite, transmitted through the egg and through contaminated food. His remedy — separate the healthy eggs from the infected, and enforce hygiene in the rearing rooms — saved the industry. The silkworm campaign was Pasteur's apprenticeship in infectious disease: it taught him to think of an epidemic as a chain of transmission that could be broken.
The same logic, applied to animals, produced the first vaccines of the germ theory era. Pasteur and his collaborators weakened — attenuated — the microbes that cause chicken cholera and anthrax, and showed that animals inoculated with the weakened germs survived later exposure to the full-strength disease. The public demonstration at Pouilly-le-Fort in 1881, in which vaccinated sheep survived anthrax while unvaccinated sheep died, made Pasteur world-famous. The principle was established: a weakened germ can teach the body to defend itself.
Pasteur's greatest trial was rabies — hydrophobia — a disease that was almost always fatal once symptoms appeared, and that terrified the public because it came from the bite of a dog. Working with the rabies virus, which he could not see under the microscope, Pasteur attenuated it by drying the spinal cords of infected rabbits, and built a course of injections of increasing strength. In July 1885, a nine-year-old boy named Joseph Meister was brought to Paris, badly bitten by a rabid dog. Pasteur, who was not a physician, hesitated — but the boy would certainly die without treatment. Pasteur treated him. The boy lived.
The news electrified the world. Within months, sufferers were travelling to Paris from across Europe and beyond, and Pasteur's laboratory became a pilgrimage site. The rabies vaccine was also a conceptual leap: it proved that vaccination could work after exposure, and that it could be extended beyond the single disease — smallpox — where Edward Jenner had pioneered it a century earlier. Vaccination was now a general method, and Pasteur's institute would become its engine.
Public subscription raised the funds for the Institut Pasteur, which opened in Paris in 1887 as a research institute and treatment centre devoted to infectious disease. It was the model for the modern biomedical research institute: laboratory science, clinical treatment, and teaching under one roof. Pasteur directed it until his death in 1895, at the age of 72. He is buried in a crypt beneath the institute — a scientist entombed in his own laboratory, as he wished.
Pasteur's legacy is not a single discovery but a transformation of the relationship between humanity and the invisible world. He proved that microbes cause fermentation, spoilage, and disease; that they can be killed by heat, excluded by hygiene, and tamed by vaccines. His rivals — above all Robert Koch in Germany, who isolated the germs of anthrax, tuberculosis, and cholera — completed the germ theory's conquest of medicine. But it was Pasteur who made the invisible visible, and who showed the world that the greatest killer in human history could be fought with a microscope, a flask, and a disciplined mind.
- Encyclopaedia Britannica, “Louis Pasteur”.
- The Institut Pasteur, Paris, official history and archives.
- Geison, Gerald L., The Private Science of Louis Pasteur.
- The Nobel Prize organisation, essays on Pasteur and the germ theory.
- The Science Museum, London, histories of pasteurization and vaccination.