As others have already said, the water you choose to drink is probably key. Few things more off putting than drinking from a stream to spot a sheep relieving itself 20 yards upstream.

Personal hygiene is also worth considering on this subject, no good the water being beutifully clean if you've had a toilet stop and the bacteria are already on you. There is also the issue of scrabbling up a steep climb and grabbing a handful of animal toilet in the process.

All this said, I've never experienced any problems. Although if I go somewhere where there is more human/animal activity (the Lakes being a good example) I'll sometimes take a mini water filter (straw type) if it seems like a good idea!