In reply to alasdair19: Get well soon guys. So sorry to hear you have got sick and thanks for the warning.
Just throwing my 2 penneth into the ring as one does on these forums!
My comments are not aimed at anyone, just sharing experiences.
I work in the Welsh hills and taking young people out for walking + camping expeditions. Mostly not in the honey pots of the North of the park more often in the south of Snowdonia. We have sheep and bracken an so on but a lot less tourists it's quite remote in fact.
I drink my water straight from a huge range carefully selected streams at least 4 days a week every week, spring, summer and autumn and rarely purify it. I choose streams with a good flow, running over a drop, and that have only just come into existence within the last few hundred meters and hence don't run from, farms, settlements, footpaths, climbing crags, campsites or lakes.
I have never ever got sick from doing this for quite a few years.
Personally I think people are far more likely to pollute the water than a few hill sheep even if there is a dead one around.
Obviously as I have no proper evidence for this belief so I get my students to take proper precautions but I often feel they miss out on the taste of what is really a very clean environment.
(I would not put anywhere on the Snowdon Massif in that category thanks to it's popularity)