The evidence for evolutionary control of the harmfulness of diarrheal pathogens is at a more advanced state than the evidence for vectorborne pathogens. Evolutionary considerations suggest that provisioning of clean water supplies should cause an evolutionary reduction in the harmfulness of diarrheal pathogens.
Indeed, much of the evidence that waterborne transmission causes evolutionary increases in harmfulness comes from studies in which the provisioning of clean water supplies was associated with a replacement of harmful diarrheal pathogens with similar but milder pathogens.
The most harmful agents of bacterial dysentery, for example, were replaced like clockwork by milder species in country after country as water supplier were purified. These studies indicate that the same trends would occur in poorer countries if sufficient investment was made in cleaning up water supplies.
Does this change occur on a finer level within particular species of pathogens?
If so, would the time period be sufficiently short to allow this evolutionary change to be incorporated into a control strategy? In theory such changes could be stronger and more rapid, the more similar bacteria are, the greater the intensity of competition between them, particularly because the cross reactive immune response becomes stronger as pathogens become more similar.