I don't think I can add much to the answers given. Just a slightly tangential thought. Can people develop a degree of resistance to water borne pathogens that they are regularly exposed to? You know...locals say the water is fine, they drink it all the time...tourist drinks it, gets sick?
I echo this thinking - depends on how tempered ones biological system is to nasties.
I was raised on a farm with a Windmill pumping water into the house - no filtration system that I am aware of.
Anyway , one time the water went 'murky' for a while the water went 'murky' in colour for a bit - we carried on drinking it.
Upon inspection later we found a Hedgehog corpse that had decided to somehow get lodged in the pipe and died. Body acting like a dirty filter..
I don't remember being sick.