Back to the original point about Giardia. Bill, the CDC reference you gave did a very good job pointing out that most cases of Giardia occur in places other than backpacking. Day care centers, public swimming pools, recreational water areas etc. I think backpacking was about 5th on the list.

I recall in the 70's when the Giardia problem started. Backpackers in Colorado were warned to treat their water. Then it spread all over and most people now treat or filter their water. It seems to be working pretty well because not many backpackers get it.