Diarrhoea is a common side effect of many feco-orally transmitted bacterial infections. How does diarrhoea help the pathogen? Should it not have a selective evolutionary advantage? Do all symptoms of the disease need to have a selective advantage for the pathogen? All I can think of is that a more liquidy stool would lead to further ease in feco-oral transmission. Is this true?

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    $\begingroup$ It would increase the overall transmission rate, I guess. Diarrhoea is not just liquidy stools (which may be better for dissemination of the bacteria) but also frequent stools (high transmission) + low digestive potential of the gut (which also protects the bacteria). You can have a look at some of the epidemiological models for cholera. $\endgroup$ – WYSIWYG Apr 11 '16 at 5:15
  • $\begingroup$ @WYSIWYG "low digestive potential " Not sure that changes anything for the bacteria since gut pathogens are adapted to live in that environnement. Unless I misinterpreted the way it could help them. $\endgroup$ – Dart Feld Nov 24 '16 at 16:24
  • $\begingroup$ Other than that you are totally correct and it is the actual answer to the question $\endgroup$ – Dart Feld Nov 24 '16 at 16:25

Vibrio cholera for example is actively causing diarrhoea, by stimulating the CFTR channel to excrete chlorine ions, which will attract water from the gut tissue.

The answer of the function seems obvious: to promote dispersal in the environment and as such infect/find new hosts. See also: extended phenotype (Richard Dawkins): how genes in one organism (mostly a parasite) influence the behaviour of another organism (mostly the host of the parasite) to increase the (reproductive) success of the gene/parasite. Also: numerous (some horrifying) examples in Parasite Rex (by Carl Zimmer)

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