What happened to Koch's postulates in diarrhoea?

In 1890, Robert Koch has formulated postulates describing what criteria a parasite has to fulfil to qualify as an aetiological agent for an infectious disease. Since then Koch's postulates have experienced reformulations by nearly every generation of microbiologists reflecting new discoveries changing the understanding of infectious diseases pathogenesis. The latest addition to this discussion is the role of the host commensal microbiota in turning infections into disease. After an overview of the historical developments of the postulates, data on diarrhoea aetiology from Bangladesh with respect to Koch's postulates were analysed. In countries with a low environmental hygiene standard, some recognized bacterial enteropathogens appear as a necessary, but not sufficient condition for diarrhoea. The possibility emerges that the loss of a physiological commensal gut microbiota equilibrium ('dysbiosis') is an important co-factor for some bacterial pathogens to induce diarrhoea. Koch's hypothesis '1 pathogen + 1 host = 1 disease' is therefore better formulated as 'X (pathogen/s) + Y (local milieu) + Z (individual host susceptibility) = disease'.