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Louis Pasteur develops a method of attenuating a virulent pathogen the agent of chicken cholera, so it would immunize and not cause disease. This is the conceptual breakthrough for establishing protection against disease by the inoculation of a weakened strain of the causative agent. Pasteur uses the word ”attenuated” to mean weakened. As Pasteur acknowledged, the concept came from Edward Jenner’s earlier success at smallpox vaccination.
Plasmids, Pasteur, and Anthrax, ASM News 49,1983. p.320 [pdf]
Pasteur, L. 1880. De l'attenuation du virus cholera des poules. Compte rend. Acad. se. 91: 673-680 In Milestones in Microbiology: 1556 to 1940, translated and edited by Thomas D. Brock, ASM Press. 1998, p126 [pdf]