From a surprising article in the Los Angeles Times:
But what he’s uncovered already is surprising, both for the seasoned wilderness traveler as well as the day hiker who stares longingly at a gushing river and wonders whether it’s safe to take a slug. At many trails and backcountry camps throughout California, signs warn visitors off casual sipping. But are the dangers of Giardia lamblia, E. coli, Cryptosporidium and other bugs that wreak intestinal havoc grossly exaggerated?
Derlet thinks so, and his research reveals that the water is much cleaner than most people believe. His findings thrust him into the middle of a long-simmering controversy that’s blatantly at odds with what many state biologists preach and what wilderness classes teach: Purify water before drinking. But is that really necessary? Do those high-priced pumps, chemical disinfectants and elaborate filtration gadgets truly merit a place in the backpack?
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The available scientific evidence, which is admittedly limited because of the scarcity of funding for testing wilderness water quality, confirms Derlet’s findings. The threat is comparable to the chances of beachgoers being attacked by a shark, according to University of Cincinnati researchers who studied the danger giardia poses to backpackers, namely “an extraordinarily rare event to which the public and the press have seemingly devoted inappropriate attention.”