His research in chemistry led to remarkable breakthroughs in the understanding of the causes and preventions of diseases, which laid down the foundations of hygiene, public health and much of modern medicine.

He is best known to the general public for his invention of the technique of treating milk and wine to stop bacterial contamination, a process now called pasteurization.

But the chemist Antoine Jérôme Balard wanted him back at the École Normale Supérieure as a graduate laboratory assistant (agrégé préparateur).

[40] In February 1854, so that he would have time to carry out work that could earn him the title of correspondent of the Institute, he got three months' paid leave with the help of a medical certificate of convenience.

[5][49] In Pasteur's early work as a chemist, beginning at the École Normale Supérieure, and continuing at Strasbourg and Lille, he examined the chemical, optical and crystallographic properties of a group of compounds known as tartrates.

[50] Pasteur determined that optical activity related to the shape of the crystals, and that an asymmetric internal arrangement of the molecules of the compound was responsible for twisting the light.

[50] Some historians consider Pasteur's work in this area to be his "most profound and most original contributions to science", and his "greatest scientific discovery.

In 1856 a local wine manufacturer, M. Bigot, whose son was one of Pasteur's students, sought for his advice on the problems of making beetroot alcohol and souring.

[56][3] Pasteur began his research in the topic by repeating and confirming works of Theodor Schwann, who demonstrated a decade earlier that yeast were alive.

[68] Since 1853, two diseases called pébrine and flacherie had been infecting great numbers of silkworms in southern France, and by 1865 they were causing huge losses to farmers.

To settle the debate between the eminent scientists, the French Academy of Sciences offered the Alhumbert Prize carrying 2,500 francs to whoever could experimentally demonstrate for or against the doctrine.

Pasteur gave a series of five presentations of his findings before the French Academy of Sciences in 1881, which were published in 1882 as 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).

[88] He made another mistake: he began by denying the "parasitic" (microbial) nature of pébrine, which several scholars (notably Antoine Béchamp)[89] considered well established.

[98] In 1884, Balbiani,[99] who disregarded the theoretical value of Pasteur's work on silkworm diseases, acknowledged that his practical process had remedied the ravages of pébrine, but added that this result tended to be counterbalanced by the development of flacherie, which was less well known and more difficult to prevent.

[100] Being unable to conduct the experiments himself due to a stroke in 1868, [101] Pasteur relied heavily on his assistants Emile Roux and Charles Chamberland.

I ask the Academy not to criticize, for the time being, the confidence of my proceedings that permit me to determine the microbe's attenuation, in order to save the independence of my studies and to better assure their progress... [In conclusion] I would like to point out to the Academy two main consequences to the facts presented: the hope to culture all microbes and to find a vaccine for all infectious diseases that have repeatedly afflicted humanity, and are a major burden on agriculture and breeding of domestic animals.

In 1877, Pasteur had earlier directed his laboratory to culture the bacteria from the blood of infected animals, following the discovery of the bacterium by Robert Koch.

"[105] Following Pasteur's criticism, Toussaint switched to carbolic acid (phenol) to kill anthrax bacilli and tested the vaccine on sheep in August 1880.

In early 1881, his laboratory discovered that growing anthrax bacilli at about 42 °C made them unable to produce spores,[110] and he described this method in a speech to the French Academy of Sciences on 28 February.

To this news, veterinarian Hippolyte Rossignol proposed that the Société d'agriculture de Melun organize an experiment to test Pasteur's vaccine.

[122] Pasteur's laboratory produced the first vaccine for rabies using a method developed by his assistant Roux,[12] which involved growing the virus in rabbits, and then weakening it by drying the affected nerve tissue.

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 study in germs, Pasteur encouraged doctors to sanitize their hands and equipment before surgery.

In 1839, Justus von Liebig, Friedrich Wöhler and Jöns Jacob Berzelius stated that yeast was not an organism and was produced when air acted on plant juice.

[62] In 1855, Antoine Béchamp, Professor of Chemistry at the University of Montpellier, conducted experiments with sucrose solutions and concluded that water was the factor for fermentation.

[150] On 12 July 1880, Toussaint presented his successful result to the French Academy of Sciences, using an attenuated vaccine against anthrax in dogs and sheep.

[128] However, Pasteur executed vaccination of the boy under the close watch of practising physicians Jacques-Joseph Grancher, head of the Paris Children's Hospital's paediatric clinic, and Alfred Vulpian, a member of the Commission on Rabies.

For example, in the US: Palo Alto and Irvine, California, Boston and Polk, Florida, adjacent to the University of Texas Health Science Center at San Antonio; Jonquière, Québec; San Salvador de Jujuy and Buenos Aires (Argentina), Great Yarmouth in Norfolk, in the United Kingdom, Jericho and Wulguru in Queensland, Australia; Phnom Penh in Cambodia; Ho Chi Minh City and Da Nang, Vietnam; Batna in Algeria; Bandung in Indonesia, Tehran in Iran, near the central campus of the Warsaw University in Warsaw, Poland; adjacent to the Odesa State Medical University in Odesa, Ukraine; Milan in Italy and Bucharest, Cluj-Napoca and Timișoara in Romania.

The official statute was registered in 1887, stating that the institute's purposes were "the treatment of rabies according to the method developed by M. Pasteur" and "the study of virulent and contagious diseases".

[187]The Literary Digest of 18 October 1902 gives this statement from Pasteur that he prayed while he worked:[188] Posterity will one day laugh at the foolishness of modern materialistic philosophers.

[55] He was given a state funeral and was buried in the Cathedral of Notre Dame, but his remains were reinterred in the Pasteur Institute in Paris,[201] in a vault covered in depictions of his accomplishments in Byzantine mosaics.