In some circumstances,untreated water may be better than no water at all.

Dehydration has serious consequences, some even fatal since lack of water increases blood density, enhancing the formation of blood clots which can travel to the heart, lungs, or brain with very bad outcomes, including death.

Of course, it is much better to always have the means to treat water, but sometimes that doesn't happen. An episode of the trots, while unpleasant and disabling, is much preferable to a heart attack or lung embolism.  There are other pernicious results from dehydration, including reduced mental capability (never a good idea when out in the woods).

When younger, during the 1950s and 60s, I quite regularly drank untreated water from various sources.  I never experienced adverse effects, but that was a time when trail use was significantly less than today.  I am much more cautious now, but there were times when I drank copiously from raw water sources.  Once you get back to town, medical help is available.

You can reclaim really bad water.  Once, doing a project in Canyon del Muerto (charming name!), we lived for a month on local stream water, which we were sure was polluted - people without sanitary facilities and herds of sheep upstream - by letting the water settle for 24 hours, decanting the clear upper layers, and then boiling.  Results were good.

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