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How did Louis Pasteur’s idea that virulence can change affect the study of infectious diseases?

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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’s recognition that virulence is a variable property—rather than a constant attribute—fundamentally transformed the study of infectious disease. By demonstrating that pathogens could be attenuated to create vaccines, he established the foundation for immunology and preventive medicine.12

Beyond attenuation, Pasteur was the first scientist to recognize that virulence could also be increased through repeated passages through different host species. He theorized that this mechanism explained the emergence of new contagions and the sudden appearance of epidemics.32

This insight remains a cornerstone of modern microbiology and epidemiology. It is critical for understanding how diseases evolve and jump between species, providing a vital framework for studying the emergence of contemporary health crises such as acquired immunodeficiency syndrome, bovine spongiform encephalopathy, and severe acute respiratory syndrome.32