Good post, aimless.

Giardia has been known literally since the invention of the microscope by van Leeuwenhoek, in the 17th century. It wasn't recognized as a human pathogen until the 20th century. Today, it's still difficult and expensive to reliably diagnose, via one of these options:

-Enteroscopy
-Stool antigen test to check for Giardia
-Stool ova and parasites exam
-String test (rarely performed)

I don't know how definitive these tests are nor what the false positive rates might be. Some people respond badly to treatment, and doing so as a cautionary measure based on symptoms w/o diagnosis carries some risk.

Giardia is but one waterborne pathogen. There's crypto, which is more dangerous and not widely considered treatable. I'm more wary of bacteria than either cyst, myself, and then there are viruses. They aren't a widespread concern in North America but nevertheless, there have been outbreaks, such as norovirus in the Grand Canyon (the NPS has a report for anybody curious).

Water treatment tech has become so easy it's simply not much of a bother to take the extra step with any source you have even a sliver of doubt about. If the concern instead is you might be lining the pockets of unscrupulous, fear-mongering, parasitic marketeers, then use somebody else's. If the concern is recovering the several hours one might have expended treating water that didn't require treatment ("opportunity cost" from another thread!) then consider chemical or gravity systems that are hands-off.

What's not kosher, in my book, is counseling others to abandon accepted backcountry practices based on one's own belief system. Assume just one in fifty water sources is contaminated. Now tell me which one it is, so I can be sure to have treatment handy. See how that might not work?

Cheers,