[A matter of methods: the historicity of Koch's postulates 1840-2000].

This paper analyses the historical origins and the popularity of 'Koch's Postulates'. In 1884 Friedrich Löffler wrote down the well-known three steps of isolation, cultivation and inoculation as conditions for establishing the existence of a pathogen. These postulates are frequently invoked in textbooks of medical history. Yet they seem to have had little relevance in medical research. Their assumed inventor, Robert Koch, produced numerous variations in his own methodology. However, underlying his work was a sort of trivial ontology of diseases which rendered an experimental reconstruction of human pathologies in animal models practical and meaningful. There were many ways to pursue this end. Koch usually limited his discussion to practical questions related to the course that investigations had to take, while matters of principle were only treated implicitly in his writings. Löffler's achievement was to popularise Koch's views in his postulates. Given that, it is not surprising that the countless references to Koch's postulates which one finds in the 20th century usually refer to the spirit rather than the literal meaning of the postulates. For example, proponents of virology or molecular medicine devise variations of Koch's postulates that serve to relate their own work to Koch's bacteriology. The latter is perceived as the origin of modern experimental medicine. The nature of such references is less historical than anecdotal: referring to a historical object that did not exist as such, these references produce ex traditione credentials for experimental medicine.