People did not need to know why water could be bad for them to avoid it. It was mainly an urban problem, of course, in general country people had wells or springs or even fairly clean rivers, and one peculiarity of London was the tides, which could mean with the ebb and flow of 30 miles or so each tide, the nett movement of the water towards the sea was constant but in practice the same water could be in central London for several days before passing on. Getting filthier all the time. Even children drank small beer.
Humans have evolved to reject poor quality water if possible, one so highly evolved as Burnaby can now detect poor quality beer, a sure survival trait.