Quite a few years ago the conventional widsom was that nearly every stream in North America was contaminated with giardia. I probably got that from Backpacker magazine or some place similar. I don't know if there is better information now, but I assume all water is dirty.

I caught something once south of the border. I don't know if it was giardia but IIRC the symptoms are similar. It stayed with me for about 3 months. I lost 20 or 30 pounds, and not just dehydration. I assume that I lost weight because food passed through me too fast to digest - I was taking Alegra sinus pills then, and I would see them whole, not broken down at all, in the toilet. My doctor had me eat lots of fiber and let it work itself out.

Nearly any method of purification is easier than finding a toilet every 20 minutes for months.