How did Louis Pasteur develop an anthrax vaccine?

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Painting by Albert Edelfelt showing Louis Pasteur in his laboratory, holding a glass flask and examining it intently, surrounded by scientific equipment on a workbench. — Louis Pasteur In the 19th century Louis Pasteur's work established the germ theory of disease, proving that microorganisms cause fermentation and sickness, which led to the development of pasteurization, aseptic techniques in medicine, and effective vaccines for diseases such as rabies. (Painting by Albert Edelfelt, 1885.)

Louis Pasteur developed the anthrax vaccine by applying the principle of attenuation, which he had previously discovered while studying chicken cholera. After confirming that the anthrax bacillus was responsible for the disease, he determined the specific conditions required to reduce the organism's virulence.1

In 1881, Pasteur conducted a famous public experiment at Pouilly-le-Fort to demonstrate the vaccine's efficacy. The procedure involved two inoculations administered 12 days apart:
* First dose: A vaccine derived from a low-virulence culture.
* Second dose: A vaccine derived from a more virulent culture than the first.

Two weeks after these vaccinations, both the immunized animals and a control group were exposed to a fully virulent strain of anthrax. All the control sheep died, while every vaccinated animal survived. This successful trial provided definitive evidence for his immunization methods and helped solidify the germ theory of disease within the medical community.1