Contributor George Novak normally hikes without water bottle or filter. He “prehydrates” — then drinks as much as he can if and when he finds good water. He has no more stomach trouble than the most careful hiker.
Everyone agrees the literature is hyper-cautious when it comes to water treatment.
Doc’s Rules for Sipping the Waters
Study the watershed you are in. Know what is there.
Look for water near to its source.
Try to take water from the sideslope streamlets.
Avoid water from the main valley stream.
Look for icy cold water.
Look for fast-moving water.
Study the area for the presence of large animals.
Ascertain whether numbers of elk have recently been in the area.
Avoid waters near beaver ponds or cattle grazing.
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Two-Heel Drive: September 2006 Archives
How careful are you with water treatment?
besthike.com 
Careful when I can, don’t worry about it when I can’t: Flagyl is easy enough to get.