[The explicative power of the rules: the case of the Koch's postulates].

This article traces the emergence of the standard for evidence of causation in infectious diseases, from the scientific discoveries on contagion by Agostino Bassi, in the first half of the nineteenth century. However, only after the work of Robert Koch on anthrax and tuberculosis, a set of practical guidelines (Koch's postulates) were acknowledged, to establish rigorous criteria before a microbiologist could claim a strictly causal relationship between microbes and infectious diseases. The methodological and epistemological meaning of these guidelines, and their limits, are discussed in the light of the recent development of microbiology and medicine.