Louis Pasteur
Germ Theory, Pasteurisation, and the Rabies Vaccine
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The book traces Pasteur's career from early education through his famous controversies. Chapters cover his early mistakes, his victory over pébrine in silkworms, and his work with flacherie disease. He developed vaccines for chicken cholera and anthrax, then successfully treated a boy with rabies using his vaccine. His laboratory notebooks later drew criticism from modern scientists.
This detailed account of Pasteur's life shows how one scientist changed medicine forever. His work on fermentation, germ theory, and vaccination laid the foundation for modern microbiology. Anyone interested in medical history or scientific breakthroughs will find this biography essential reading.
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Louis Pasteur arrived on 27 December 1822 in Dole, Jura, France, into a Catholic household where his father worked as a tanner. His parents were named Jean-Joseph Pasteur and Jeanne-Etiennette Roqui. The family relocated first to Marnoz in 1826, and then to Arbois the following year. By 1831, Pasteur had begun his primary schooling. He struggled with dyslexia and dysgraphia throughout his early years.
Louis Pasteur didn’t stand out as a student early on, and his interests lay more in fishing and sketching than in books. He spent time drawing pastels and portraits of his parents, friends, and neighbors. Later, he went to the Collège d'Arbois for secondary school. In October 1838, he traveled to Paris to enter a boarding school, but after only a few weeks, homesickness brought him back in November.
In 1839, Pasteur entered the Collège Royal at Besançon, where he studied philosophy and earned his Bachelor of Letters degree the following year. He was then appointed as a tutor at the same college while continuing his science studies with a focus on mathematics. His first examination in 1841 did not go well, but he later passed the baccalauréat scientifique degree from Dijon. There, he received his Bachelor of Science in Mathematics, known as Bachelier ès Sciences Mathématiques, in 1842, though his performance in chemistry was only average.
After struggling through the entrance exam for the École Normale Supérieure in 1842—feeling most at ease with physics and math—Pasteur passed the initial round but decided to try again the following year. He returned to a Parisian boarding school to prepare, while also attending classes at the Lycée Saint-Louis and listening to lectures by Jean-Baptiste Dumas at the Sorbonne. The next year, 1843, he succeeded with a strong ranking and entered the school. There, he studied under Jean-Baptiste Boussingault at the Conservatoire national des arts et métiers and earned his licencié ès sciences in 1845. The following year, he became a professor of physics at the Collège de Tournon in Ardèche, but chemist Antoine Jérôme Balard brought him back to the École Normale Supérieure as a graduate laboratory assistant. He began research in crystallography and in 1847 submitted two theses: one in chemistry titled Recherches sur la capacité de saturation de l'acide arsénieux, and another in physics about polarisation rotatoire.
After a short time as professor of physics at the Dijon Lycée in 1848, Pasteur took up a position as professor of chemistry at the University of Strasbourg. There, in 1849, he met and began courting Marie Laurent, who was the daughter of the university’s rector. They married on 29 May 1849. Together, they had five children, though only two lived to adulthood; the other three died from typhoid.
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Louis Pasteur began his academic career in 1848 when he was appointed professor of chemistry at the University of Strasbourg. Three years later, in 1852, he became the chair of chemistry at the same institution. These positions marked important steps in his development as a scientist, setting the stage for his later groundbreaking work in germ theory, pasteurization, and vaccine development.
In February 1854, Louis Pasteur arranged for three months of paid leave with the help of a medical certificate, extending it until August 1st so he could take the exams that would earn him the title of correspondent of the Institute. He told the Minister he would go and do the examinations to avoid increasing the service's embarrassment and also to prevent leaving another person responsible for a sum of 6 or 700 francs.
In 1854, Louis Pasteur became dean of the new faculty of sciences at the University of Lille, where he started researching fermentation. It was during this time that he said, "dans les champs de l'observation, le hasard ne favorise que les esprits préparés," which means, "In the field of observation, chance favors only the prepared mind."
In 1857, Louis Pasteur became the director of scientific studies at the École Normale Supérieure in Paris. He took control in 1858 and led reforms that raised the standards of scientific work. The exams grew stricter, which improved results and made the school more competitive and respected. But his methods were often rigid and authoritarian. During what came to be known as "the bean revolt," he ordered that a mutton stew, which students had refused to eat, be served and eaten every Monday. At another point, he threatened to expel any student caught smoking. Of the 80 students in the school, 73 resigned.
In 1863, Pasteur began teaching at the École nationale supérieure des Beaux-Arts, where he worked until 1867. That year, he also took over the chair of organic chemistry at the Sorbonne, though his health soon grew poor. Also in 1867, Pasteur requested and received approval to create the École Normale's laboratory of physiological chemistry, a role he fulfilled for more than two decades, from 1867 to 1888. Later that same year, he founded the Pasteur Institute in Paris and led it for the rest of his life.
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In his early days as a chemist, Pasteur worked at the École Normale Supérieure, and later in Strasbourg and Lille, where he studied tartrates— a group of chemical compounds. He looked closely at their chemical, optical, and crystallographic properties.
In 1848, he solved a puzzle about tartaric acid. A solution from living things twisted light's plane of polarization, but chemically made tartaric acid didn’t have that effect, even though it had the same chemical reactions and same elements.
Pasteur observed that tartrate crystals often had small faces, and when he studied racemic mixtures more carefully, he saw that half were right-handed while the other half were left-handed. In solution, the right-handed form rotated light to the right, called dextrorotatory, while the left-handed one rotated it to the left, known as levorotatory. He concluded that the twisting of light resulted from the shape of the crystals and linked this to an asymmetric internal structure within the molecules. The (2R,3R)- and (2S,3S)- tartrates were identical in form but could not be superimposed on each other, making them mirror images. This discovery marked the first demonstration of molecular chirality and provided the first clear explanation of isomerism.
Pasteur’s contributions in this field have been described by some historians as his most profound and original scientific work, calling it his greatest discovery.
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Pasteur began looking into fermentation while working in Lille. In 1856, a local wine maker named M. Bigot asked for his help with issues involving beetroot alcohol and souring. Pasteur started his study by repeating and confirming the work of Theodor Schwann, who had shown earlier that yeast were alive.
In August 1857, Pasteur sent a paper on lactic acid fermentation to the Société des Sciences de Lille. It was read three months later, and a memoire followed on 30 November 1857. In that memoire, he laid out his ideas, stating: "I intend to establish that, just as there is an alcoholic ferment, the yeast of beer, which is found everywhere that sugar is decomposed into alcohol and carbonic acid, so also there is a particular ferment, a lactic yeast, always present when sugar becomes lactic acid."
In 1858, Pasteur published his complete memoir on alcoholic fermentation, directly challenging earlier theories from scientists such as Jöns Jacob Berzelius and Justus von Liebig, who thought fermentation occurred through decomposition. He demonstrated instead that yeast was the agent responsible for converting sugar into alcohol. His work also revealed that when wine was contaminated by a different kind of microorganism, lactic acid formed, causing the wine to turn sour. Then, in 1861, he noticed a reduction in the amount of sugar fermented for each portion of yeast when the yeast was exposed to air—a phenomenon that would later be called the Pasteur effect.
Pasteur discovered that micro-organisms caused beverages like beer, wine, and milk to spoil. To fight this, he developed a method where liquids were heated to between 60 and 100 degrees Celsius. This killed most bacteria and moulds already present. He worked with Claude Bernard, who helped test blood and urine on April 20, 1862. Pasteur patented the process in 1865 to combat wine diseases. The technique became known as pasteurization and was soon used on beer and milk.
Pasteur’s work with fermenting liquids showed him that tiny organisms were responsible not just for spoiling drinks, but also for causing illness in animals and people. This discovery led him to suggest that preventing these micro-organisms from entering the body could stop disease. His ideas influenced Joseph Lister, who later developed antiseptic techniques in surgery to avoid infections during operations.
In 1866, Pasteur published Études sur le Vin, a study on the diseases affecting wine. Then, in 1876, he released Études sur la Bière, which looked at the illnesses that attacked beer. These works were part of his broader investigation into fermentation and the role of microorganisms in causing spoilage and disease.
In the early 1800s, Agostino Bassi proved that a fungus caused muscardine, a disease affecting silkworms. By 1853, two new diseases—pébrine and flacherie—had begun killing huge numbers of silkworms in southern France. These illnesses were causing devastating losses for farmers by 1865. That year, Pasteur traveled to Alès, where he worked for five years until 1870.
Pasteur studied silkworm diseases and found that pébrine was caused by corpuscles, which he initially thought were a symptom but later concluded were the actual cause—though we now know it's a microsporidian. He showed the disease could be hereditary and created a method to prevent it: after female moths laid eggs, the moths were turned into pulp, examined under a microscope, and if corpuscles were found, the eggs were destroyed. He also determined that bacteria caused flacherie, though today we know viruses are primarily responsible. Flacherie could spread accidentally or be inherited, and hygiene helped prevent accidental cases. By using moths without the disease-causing microorganisms in their digestive systems to lay eggs, Pasteur was able to stop hereditary flacherie.
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After his work with fermentation, Pasteur proved that the skin of grapes naturally carried yeasts, which caused wine-making. He took grape juice from under the skin using sterilized needles, and also covered grapes with sterilized cloth. In both cases, when the juice was placed in sterilized containers, it never fermented. These experiments showed that fermentation came from outside sources, not from within the grapes themselves.
Pasteur’s work challenged the widely accepted idea of spontaneous generation, which held that life could arise from non-living matter. His ideas directly contradicted those of Félix Archimède Pouchet, who was the director of the Rouen Museum of Natural History. The debate between these two prominent scientists captured the attention of the French Academy of Sciences. To resolve the dispute, they offered a prize known as the Alhumbert Prize, with a cash award of 2,500 francs. The contest was open to anyone who could provide experimental proof for or against the theory of spontaneous generation.
In the late 1850s, Pouchet claimed that air itself was responsible for the spontaneous generation of living organisms in liquids, and he published experiments to support this idea. Earlier, in the 17th century, Francesco Redi had challenged the concept, and later, in 1765, Lazzaro Spallanzani conducted experiments suggesting that air contaminated broths with bacteria. By the 1860s, Pasteur had repeated Spallanzani’s work, but Pouchet's results differed when he used a different type of broth.
Pasteur conducted a series of experiments to challenge the idea that life could arise from non-life. He boiled liquid in flasks and allowed hot air to enter, then sealed them—no organisms grew. In another test, when he opened flasks with boiled liquid, dust entered and caused some to develop organisms. At higher altitudes, fewer flasks showed growth, indicating less dust in the air. He also used swan neck flasks with curved tubes that trapped dust, preventing contamination. Nothing grew in the broth unless the flask was tilted, allowing the liquid to touch the dusty neck. These results proved that organisms came from outside sources, carried on dust, not from within the liquid or pure air itself.
Pasteur delivered a series of five presentations to the French Academy of Sciences in 1881, which were later published in 1882 under the title Mémoire Sur les corpuscules organisés qui existent dans l'atmosphère: Examen de la doctrine des générations spontanées (Account of Organized Corpuscles Existing in the Atmosphere: Examining the Doctrine of Spontaneous Generation). He was awarded the Alhumbert Prize in 1862 for his work. In these experiments, he concluded that “Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.” He stated there was no known circumstance in which microscopic beings could come into the world without germs, without parents similar to themselves.
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When Pasteur arrived in Alès, he began studying pébrine, a silkworm disease, along with another one called flacherie, also known as dead-flat disease. Unlike Quatrefages, who had recently coined the term pébrine, Pasteur initially thought the two illnesses were the same. In fact, he believed most of the silkworm diseases known up to that point were identical to each other and to pébrine. It wasn’t until letters he wrote on 30 April and 21 May 1867 to Dumas that he finally made the distinction between pébrine and flacherie.
Pasteur’s early work included a significant misstep when he rejected the microbial cause of pébrine, a silkworm disease that Antoine Béchamp and others had already accepted as clearly linked to parasites. In August 1866, a note from Balbiani initially seemed to support Pasteur’s position, but it changed nothing immediately. One observer remarked, “Pasteur is mistaken,” adding that Pasteur would not revise his view until sometime during 1867.
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When Pasteur still hadn’t figured out what caused the pébrine, he created a way to fight the disease threatening the silk industry in the Cévennes. He took a sample of chrysalises, crushed them, and looked for corpuscles in the resulting mixture. If only a small number of the pupae contained these corpuscles, the chamber was deemed suitable for breeding. This technique of selecting “seeds” — or eggs — resembled a method Osimo had suggested a few years earlier, although Osimo’s tests had not worked out. Pasteur’s approach stopped the pébrine and preserved much of the local silk production.
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In 1878, at the Congrès international séricicole, Pasteur spoke about the challenges of combating silkworm diseases. He admitted that while pébrine might be overcome, flacherie still continued to cause damage. He believed this was because farmers had not followed his advice.
In 1884, Balbiani, who had dismissed Pasteur’s theoretical contributions to understanding silkworm diseases, admitted that Pasteur’s practical method had successfully treated the damage caused by pébrine. But Balbiani also pointed out that this success was often offset by another problem known as flacherie. He noted that flacherie was less familiar and harder to prevent than pébrine had been.
Even though Pasteur managed to fight off pébrine, the silk industry in France still took a heavy hit. The country’s sericulture faced serious losses, and despite his efforts to address one of its biggest problems, the damage done could not be reversed. Pébrine had devastated silkworms, and while Pasteur found a way to deal with the disease, it wasn’t enough to save the broader sector from decline. French silk production continued to suffer in the face of these ongoing challenges.
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Pasteur's early vaccine work began with chicken cholera, using bacteria samples from Henry Toussaint—later named Pasteurella multocida in his honor. After a stroke in 1868 left him unable to work directly, he depended on assistants Emile Roux and Charles Chamberland. Experiments started in 1877, and by 1878, Roux had successfully grown a stable bacterial culture in broths. In October 1879, delayed by his daughter's wedding and illness, Pasteur told Roux to begin a new culture from one that had been sitting since July. The two chickens inoculated with this new batch showed signs of infection but recovered. After incubating the culture for eight more days, Roux inoculated the same chickens again, and this time they died. As Pasteur later noted in his notebook, these results showed how bacteria could be weakened in lab conditions, pointing toward a way to create vaccines through attenuation.
In February 1880, Pasteur presented his findings to the French Academy of Sciences under the title "Sur les maladies virulentes et en particulier sur la maladie appelée vulgairement choléra des poules," or "On virulent diseases, and in particular on the disease commonly called chicken cholera." He published the work in the academy's journal, Comptes-Rendus hebdomadaires des séances de l'Académie des Sciences. He explained that bacteria lost their virulence when exposed to air during cultivation, while those kept in sealed containers retained their power. Pasteur introduced the term "attenuation" for this process and said: “We can diminish the microbe's virulence by changing the mode of culturing. This is the crucial point of my subject.” He asked the Academy not to criticize his methods yet, so he could continue his work freely. In conclusion, he pointed out that his findings opened the door to cultivating all microbes and developing vaccines for infectious diseases that harm both humans and animals.
Pasteur's work with chicken cholera included developing a vaccine that did not reliably create immunity. Later studies proved this early vaccine was not effective, though initial attempts showed promise before failing to deliver consistent protection. This setback did not stop Pasteur from continuing his research into infectious diseases and vaccines. His experiments with chicken cholera were part of a larger effort to understand how germs cause illness and how to fight them. Though this particular vaccine did not work, it helped shape future approaches to immunization. The lessons learned contributed to developing better methods for preventing disease. Even though the vaccine itself was ineffective, Pasteur's broader contributions to science were significant. His later work would prove more successful in fighting deadly diseases like rabies.
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After his work with chicken cholera, Pasteur turned his attention to anthrax, a disease that was killing cattle. In 1877, he directed his laboratory to grow the bacteria from the blood of infected animals. This followed the discovery of the bacterium by Robert Koch. Pasteur used the method he had developed for chicken cholera to create a vaccine for anthrax.
When animals became infected with the bacteria, anthrax developed, confirming the role of the microorganism in causing the disease. Cattle were dying in large numbers in areas known as "cursed fields." Pasteur was informed that sheep buried in these fields had died from anthrax. He proposed that earthworms could have carried the bacteria to the surface. His investigation revealed anthrax bacteria in the earthworms’ waste, validating his theory. As a result, he advised farmers to stop burying dead animals directly in the fields.
On 12 July 1880, Henri Bouley presented a report to the French Academy of Sciences by Henry Toussaint, a veterinary surgeon who wasn't a member of the academy. Toussaint claimed to have developed an anthrax vaccine by heating bacilli at 55 °C for ten minutes. He tested it on eight dogs and eleven sheep, half of which died after inoculation, making it a poor result. Pasteur was shocked and wrote to the academy, saying he couldn't believe dead vaccine would work and that Toussaint's claim "overturns all the ideas I had on viruses, vaccines, etc." After criticism, Toussaint switched to using carbolic acid to kill the bacilli and tested again in August 1880. Pasteur thought such a killed vaccine shouldn't work because he believed attenuated bacteria used up nutrients they needed to grow, and that oxidizing them in culture broth for long periods made them less virulent.
In early 1881, Pasteur's team discovered that anthrax bacilli differed from chicken cholera bacilli—they didn't weaken easily when cultured in air because they formed spores, but at around 42°C, bacteria lost spore-forming ability. He shared this method with the French Academy of Sciences on 28 February. By 21 March, he claimed success vaccinating sheep, though results were inconsistent. Veterinarian Hippolyte Rossignol proposed a public test through the Société d'agriculture de Melun, and Pasteur accepted on 28 April. His assistants Roux and Chamberland doubted reliability, so Chamberland secretly prepared a chemical version. They conducted the experiment at Pouilly-le-Fort in May using 58 sheep, 2 goats, and 10 cattle. Half were vaccinated on 5 and 17 May; the rest remained unvaccinated. On 31 May, all received fresh virulent anthrax culture. Official results announced on 2 June before over 200 spectators showed vaccinated animals survived while unvaccinated ones died or became very ill—exactly as predicted. In his report to the French Academy of Sciences on 13 June, he said this vaccine marked significant progress beyond Jenner's smallpox vaccine. Though he described it as a "live vaccine," his notebooks show he actually used a potassium dichromate-killed version, similar to Toussaint's method.
The idea that a milder version of a disease could protect against a more severe form wasn’t new—people had known this about smallpox for a long time. Inoculation with smallpox, known as variolation, was practiced and shown to lead to a less serious illness and much lower death rates than getting the disease naturally. Edward Jenner studied using cowpox, or vaccinia, to give protection against smallpox in the late 1790s. By the early 1800s, this method of vaccination had spread across most of Europe.
The method used for anthrax and chicken cholera vaccines was different from smallpox vaccination because these disease organisms had been artificially weakened. Pasteur developed this technique of creating weakened pathogens, which changed how infectious diseases were studied and treated. He named these weakened forms "vaccines," honoring Jenner’s original discovery. This approach meant that instead of finding a naturally weak version of the disease, scientists could create one through artificial means.
In 1876, Robert Koch demonstrated that Bacillus anthracis caused anthrax. Pasteur acknowledged his work only in a footnote within papers published between 1878 and 1880. The two scientists met at the Seventh International Medical Congress in 1881. Shortly afterward, Koch accused Pasteur of using impure cultures and committing errors. The following year, Pasteur delivered a speech in response, which prompted a harsh rebuttal from Koch. Koch asserted that Pasteur had tested his vaccine on inappropriate animals and that his research was not scientifically sound. In 1882, Koch wrote “On the Anthrax Inoculation,” where he disputed several of Pasteur’s findings. He also charged that Pasteur had failed to disclose his methods, drawn premature conclusions, and lacked precision.
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In 1882, Pasteur sent his assistant Louis Thuillier to southern France to investigate an outbreak of swine erysipelas. By March 1883, Thuillier had identified the bacillus responsible for the disease. Pasteur and Thuillier then strengthened the bacterium's virulence by passing it through pigeons, and later through rabbits, which weakened it enough to create a vaccine. At the time, they incorrectly described the bacterium as having a figure-eight shape. But in 1884, Roux described it instead as stick-shaped.
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In 1885, Louis Pasteur treated a nine-year-old boy named Joseph Meister after he was bitten badly by a rabid dog. The treatment used a vaccine developed by Pasteur’s assistant Emile Roux, which involved weakening the virus in rabbits before drying the nerve tissue. Pasteur had tested the vaccine on fifty dogs first, and Meister received thirteen inoculations over eleven days. Three months later, Meister was healthy, and no legal action followed, despite Pasteur not being a licensed doctor. Before Meister, Pasteur had treated two other people— one survived, possibly without rabies, and another died from it. Later that same year, Pasteur treated Jean-Baptiste Jupille successfully, and by 1886, he had treated three hundred fifty people, with only one developing rabies. This success led to the creation of the first Pasteur Institute.
Pasteur was absolutely fearless. Anxious to secure a sample of saliva straight from the jaws of a rabid dog, I once saw him with the glass tube held between his lips draw a few drops of the deadly saliva from the mouth of a rabid bull-dog, held on the table by two assistants, their hands protected by leather gloves.
Because of his work with germs, Pasteur pushed doctors to clean their hands and tools before surgeries. Back then, most doctors didn’t bother with such steps. Earlier in the 1860s, two men named Ignaz Semmelweis and Joseph Lister had already begun using hand sanitizing in medical settings.
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At age fifty-five, Louis Pasteur was already a French national hero when he quietly instructed his family to never let anyone see his laboratory notebooks. His family honored that request, and all his documents remained secret for decades. Because Pasteur didn’t allow lab assistants to keep their own records, much of his work stayed hidden until recently. Finally, in 1964, Pasteur’s grandson and last surviving male heir, Pasteur Vallery-Radot, donated the papers to the French national library. But the documents weren’t open for historical research until after Vallery-Radot died in 1971. And it wasn’t until 1985 that they were given a catalogue number at all.
In 1995, on the centennial of Louis Pasteur’s death, historian Gerald L. Geison examined Pasteur’s private notebooks and published an analysis in The Private Science of Louis Pasteur, where he claimed that Pasteur had given misleading accounts and engaged in deception during his most important discoveries. Max Perutz responded with a defense of Pasteur in The New York Review of Books. Later, French immunologist Patrice Debré looked more closely at Pasteur’s documents and wrote Louis Pasteur in 1998, concluding that although Pasteur was brilliant, he had flaws. A book review described Debré as sometimes finding him "unfair, combative, arrogant, unattractive in attitude, inflexible and even dogmatic."
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Before Pasteur, scientists had already begun looking into fermentation. In the 1830s, Charles Cagniard-Latour, Friedrich Traugott Kützing, and Theodor Schwann examined yeasts under microscopes and decided that yeasts were living organisms. Then, in 1839, Justus von Liebig, Friedrich Wöhler, and Jöns Jacob Berzelius claimed yeast was not an organism at all. They argued that yeast formed when air reacted with plant juice.
In 1855, Antoine Béchamp, who was Professor of Chemistry at the University of Montpellier, did experiments with sucrose solutions and decided that water was what caused fermentation. Then in 1858, he changed his mind and said that fermentation happened because of moulds growing in the liquid, and that these moulds needed air to live. Béchamp believed he was the first person to prove that tiny living things were involved in the process of fermentation.
Pasteur began his work in 1857 and shared his results the following year, in 1858. His paper appeared in April in the Comptes Rendus Chimie. A rival scientist, Béchamp, had already published related findings in January of the same year. Béchamp later claimed that Pasteur had not introduced anything new or original in his experiments. Still, Béchamp likely knew about Pasteur’s early research from 1857. Because both men insisted they deserved credit for the discovery, their disagreement continued for years, growing into a broader conflict that spanned multiple scientific areas.
The BMJ obituary noted that Béchamp was on the losing side of the debate, remarking that his name was "associated with bygone controversies as to priority which it would be unprofitable to recall." Béchamp's theory of microzymes was ultimately rejected. According to K. L. Manchester, later supporters of alternative medicine and anti-vivisectionists promoted Béchamp’s ideas, unjustifiably claiming that Pasteur had plagiarized him.
Pasteur thought that succinic acid was responsible for inverting sucrose, but in 1860, Marcellin Berthelot isolated invertase and proved that succinic acid did not cause the inversion. At the same time, Pasteur maintained that fermentation occurred only through the action of living cells. This view led to a long debate between him and Berthelot over the concept of vitalism, with Berthelot firmly opposing any notion of vitalism. Later, Hans Buchner discovered that zymase—though not a single enzyme, but rather a mixture of enzymes—was responsible for catalyzing fermentation inside cells. Eduard Buchner then showed that fermentation could also take place outside of living cells.
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In 1881, Pasteur publicly claimed success with his anthrax vaccine, but it was actually Henry Toussaint who had developed the first version. Toussaint isolated the bacteria causing chicken cholera—later named Pasteurella in Pasteur’s honor—in 1879 and shared samples with him. On 12 July 1880, Toussaint presented his attenuated vaccine against anthrax to the French Academy of Sciences, testing it on dogs and sheep. Jealousy drove Pasteur to counter this by displaying his own method at Pouilly-le-Fort on 5 May 1881. He claimed he made a “live vaccine,” though he actually used potassium dichromate to inactivate anthrax spores—similar to Toussaint’s approach. The experiment was successful and helped Pasteur gain credit and profit.
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Pasteur’s work is often judged harshly for ethical reasons, especially his decision to vaccinate a young boy named Meister. At the time, Pasteur had no medical training and no license to practice medicine. His colleague Émile Roux, who was qualified, refused to take part in the trial, likely seeing it as unethical. Still, Pasteur went ahead with the treatment under the supervision of two doctors: Jacques-Joseph Grancher, head of the paediatric clinic at Paris Children’s Hospital, and Alfred Vulpian, a member of the Commission on Rabies. Grancher actually gave the injections, while Pasteur watched closely from behind. It was Grancher who later defended Pasteur in front of the French National Academy of Medicine.
Pasteur faced criticism for not sharing his methods openly or conducting proper animal testing before human trials. He said he kept his procedure secret to maintain control over its quality and only later told a small group of scientists about it. Pasteur claimed he had successfully vaccinated fifty rabid dogs before using the treatment on young Joseph Meister. But according to Geison, Pasteur’s lab notebooks show he had actually vaccinated just eleven dogs.
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Louis Pasteur married Marie Laurent in 1849; she was the daughter of the rector of the University of Strasbourg and also served as his scientific assistant. They had five children together, but tragically, three of them died in childhood. Their eldest daughter, Jeanne, was born in 1850 and died from typhoid fever at age nine while attending the boarding school in Arbois in 1859. In 1865, their two-year-old son Camille died of a liver tumor. Shortly after bringing their daughter Cécile home from school, she too succumbed to typhoid fever on May 23, 1866, at age twelve. Only Jean Baptiste and Marie Louise survived to adulthood; Jean Baptiste later served as a soldier in the Franco-Prussian War.
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Louis Pasteur’s grandson, Louis Pasteur Vallery-Radot, noted that Pasteur had retained only a spiritualism from his Catholic upbringing, without engaging in formal religious practice. Yet others, including Catholic observers, believed he stayed an ardent Christian his entire life. His son-in-law, who wrote a biography of him, also described Pasteur’s continued faith. These differing perspectives reveal the complexity of Pasteur’s personal beliefs as understood by those closest to him.
Louis Pasteur lived with a deep and unwavering faith in God and in the promise of eternity. He believed that the good power given to him in this life would continue beyond it. These feelings shaped his entire existence, and the teachings of the gospel were always present in his thoughts. Though he respected the religious traditions of his ancestors, he turned naturally to faith during his final weeks, seeking spiritual comfort as he faced the end of his life.
As I continue my work in the lab, I find myself praying, and it's not just because I'm engaged in scientific discovery. The deeper I go into studying nature, the more I'm struck by what I see as the hand of a Creator. I believe that future generations may look back and think it strange that people once dismissed such wonder. But for now, I remain humbled in the face of what I observe, and I pray while I work.
Maurice Vallery-Radot, whose family connections included being the grandson of the brother of Pasteur’s son-in-law, was a devout Catholic who believed Pasteur stayed fundamentally Catholic throughout his life. Both he and another Maurice Vallery-Radot disputed the authenticity of a well-known quote often attributed to Pasteur: “The more I know, the more nearly is my faith that of the Breton peasant. Could I but know all I would have the faith of a Breton peasant's wife.” According to Maurice Vallery-Radot, this quotation first appeared after Pasteur died. Though Pasteur believed in God, his views were said to reflect those of a freethinker rather than a Catholic, making him more of a spiritual man than a religious one. He also opposed mixing science and religion.
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Louis Pasteur endured a serious stroke in 1868 that left him paralyzed on the left side, yet he recovered. In 1894, a second stroke or uremia struck him again, this time leaving him in failing health. He passed away on 28 September 1895, near Paris. A state funeral was held for him, and he was initially buried in the Cathedral of Notre Dame. His remains were later reinterred at the Pasteur Institute in Paris, in a vault adorned with Byzantine mosaics that illustrate his accomplishments.
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Louis Pasteur's name graces streets worldwide, testament to his enduring impact. In the US, streets named after him appear in Palo Alto and Irvine, California, Boston and Polk, Florida, near the University of Texas Health Science Center at San Antonio. Quebec's Jonquière, Argentina's San Salvador de Jujuy and Buenos Aires, the UK's Great Yarmouth, Australia's Jericho and Wulguru in Queensland, Cambodia's Phnom Penh, Vietnam's Ho Chi Minh City and Da Nang, Algeria's Batna, Indonesia's Bandung, Iran's Tehran, Poland's Warsaw near Warsaw University, Ukraine's Odesa near Odesa State Medical University, Italy's Milan, and Romania's Bucharest, Cluj-Napoca, and Timișoara all honor him. In Saigon, Vietnam, Avenue Pasteur remains one of the few streets to keep its French name. Boston's Avenue Louis Pasteur in the Longwood Medical and Academic Area follows the French tradition with "Avenue" preceding the name.
The name Pasteur lives on in institutions across the globe. The Institut Pasteur and Université Louis Pasteur stand as enduring tributes to his work. Educational spaces carry his name too, like the Lycée Pasteur in France and the Lycée Louis Pasteur in Canada. In South Africa, hospitals named after him can be found in Pretoria and Bloemfontein. Another hospital, Louis Pasteur University Hospital, is located in Košice, Slovakia. These places reflect a legacy that spans continents and continues to influence science and medicine today.
A bronze bust of Pasteur can be found on the French Campus of Kaiser Permanente's San Francisco Medical Center in San Francisco. The sculpture was designed by Harriet G. Moore and cast in 1984 by Artworks Foundry. In San Rafael, California, a statue of Pasteur stands at San Rafael High School.
The UNESCO/Institut Pasteur Medal was established on the centenary of Pasteur's death to honor exceptional research that improves human health. Given every two years in his name, it recognizes scientists whose work has made a significant difference in medicine and public health.
Henri Mondor, who held the title of French Academician, once observed that Pasteur did not practice as a doctor or perform operations, yet his contributions to the fields of medicine and surgery surpassed those of many who worked directly in these areas.
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