I am far out of my league to be able to add to the intellectual ideas bouncing aroung here, but it does bring to mind an article that I had once read that the clean water scare was just that, a scare tactic (sp?) designed to get us to buy water filters. The tests that discovered giardia in the water wasn't able to confirm that the giardia was ALWAYS in the water and that the human system was ALWAYS able to resist a certain amount of the cysts in the mountain streams. In fact, the article said that most cases of backpacking sickness was caused by poor cooking and cleaning hygiene (and personal hygiene) than because of the water.

The point being that many hikers will clean out their dishes with their tongues or fingers (to save water, weight, etc.), get sick, and then tell everybody it was the water.