With the aid of the Cahiers de laboratoire, the Correspondance and, of course, the Oeuvre de Pasteur, this work reconstructs the extraordinary scientific undertakings of the great French scientist in his study of silkworm diseases. The focus of this study consists in the attempt to explain the initial perplexing behaviour of Pasteur, even in the presence of correct interpretations regarding the causes of these diseases (cfr. the results obtained by Béchamp); for a good three years he insisted on maintaining that (...) the aetiology of silkworm diseases could not be attributed to pathogenetic germs from outside. And this was in spite of the fact that previously (through fermentation and spontaneous generation) he had been able to demonstrate the importance of micro-organisms in biological processes. Finally it is intended to highlight the extraordinary methodological depth of that initial 'error', which was capable of paving the way for the future conquests of Pasteur in the field of aetiology and the prevention of infectious diseases. (shrink)