This paper marked the beginning of Pasteur’s work on the attenuation of the infective organism. Noting that fowls inoculated with an attenuated form of the chicken cholera bacterium acquired immunity, he developed the idea of a protective inoculation by attenuated living cultures, and subsequently adopted this principle with anthrax, rabies, and swine erysipelas. His work laid the foundations of the science of immunology. Since 1979, the availability to scholars of Pasteur’s original laboratory notebooks has provided evidence that Émile Roux played a crucial and previously unacknowledged role in the development of the vaccine. See also his later paper in the same journal, 1880, 91, 673-80. Abridged English translation of both papers and discussion of Roux’s role in Bibel, Milestones in immunology (1988). Roux did receive credit from Pasteur for his work on anthrax. See No. 5169.