What happened to Koch’s postulates in diarrhoea? 

Environ. Microbiol. (2017)

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".