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?
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)