I don't think anyone in this thread has resorted to "frantic alarms about the perils of giardiasis" nor have they "aroused exaggerated concern about this infestation".

Whenever I see such highly colored language being used, it tells me that the author sees his position as adversarial, and it serves as a red flag, indicating that whatever information follows will be selected entirely on the basis of how well it substantiates the author's conclusions.

Giardiasis is not fatal. It is possible to be infected but asymptomatic. High concentrations of cysts may be very localized and infrequent. And statistically speaking, it is possible to drink untreated water from the High Sierra on a very regular basis for multiple decades and never suffer from it. All that is true.

It is also true that a nasty bout of giardiasis is extremely unpleasant and anyone who suffers through one bout of it will wish never to do so again as long as they live, whereas treatment is not especially difficult or expensive, and helps eliminate one's exposure to those infrequent high concentrations they might run across in a local water source.

Given both sides of the issue, choosing to treat is a perfectly rational choice, even without "frantic alarm" or "exaggerated concern." Not treating is also a perfectly acceptable choice, but it would be wrong to call it a choice that has no risk associated with it whatsoever, just as it would be wrong to raise "frantic alarms" over it.