Why did vaccinated sheep survive the anthrax experiment at Pouilly-le-Fort?

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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.)

The vaccinated sheep survived the experiment at Pouilly-le-Fort because they had been successfully immunized against anthrax using attenuated cultures of the bacillus.

Louis Pasteur developed a two-step vaccination procedure to prepare the animals' immune systems:

First inoculation: The sheep received a vaccine derived from a low-virulence culture.
Second inoculation: Twelve days later, they received a second vaccine made from a more virulent culture than the first.

Two weeks after these initial treatments, both the vaccinated sheep and a group of unvaccinated control sheep were exposed to a fully virulent strain of anthrax. While the control group died within a few days, the vaccinated animals survived, providing definitive proof of the effectiveness of immunization and supporting the germ theory of disease.1