Some people are more susceptible to beaver fever than others. I had a buddy, who like Redcanoe's friend, had to use filtered water to brush his teeth. I sometimes wish I could catch the dang thing, because it's one hell of a weight loss program. One of my buddies got it and lost 40 pounds in two weeks. Despite all my efforts, I haven't managed to get it yet. I remember there was a water engineer over on Myccr who claimed that filtering was a waste of time, because of all the cross contamination issues. He maintained that there was no way for the filtered water to remain unblemished because so many parts touched each other. He thought that most incidents of beaver fever on group canoe trips probably came from improper hand washing(trace amounts of fecal content) and food preparation.
I use a ceramic filter now, mostly because it makes the water taste very nice.