How did Louis Pasteur discover a vaccine for chicken cholera?

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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 discovered the vaccine for chicken cholera in 1879 through a chance observation, famously noting that "chance only favours the prepared mind."1

He observed that cultures of the bacteria responsible for the disease lost their pathogenicity over many generations, retaining only "attenuated" or weakened characteristics. By inoculating chickens with this attenuated form, he demonstrated that the birds developed resistance to the fully virulent strain of the disease. This discovery allowed Pasteur to establish the principle of immunization, which he subsequently applied to other diseases like anthrax and rabies.12