From the "old news" Rockwell article
Quote:
Ask the average outdoors person about Giardia lamblia or giardiasis, and they have certainly heard about it. Almost always, however, they are considerably misinformed about both the organism’s prevalence in wilderness water, and the seriousness of the disease if contracted.
With the advent of the Internet, the amount of information one can easily find on the subject is voluminous. Unfortunately, most of it is flawed in important aspects, being unsubstantiated, anecdotal, or merely quoting other unsubstantiated and anecdotal articles. Official sources, such as many informational publications put out by the US government, are not immune to this criticism.

This paper is the result of a critical distillation of relevant articles, retaining only those from scholarly, peer-reviewed, or otherwise professional and trustworthy sources.

One conclusion of this paper is that you can indeed contract giardiasis on visits to the Sierra Nevada, but it won’t be from the water. So drink freely and confidently: Proper personal hygiene is far more important in avoiding giardiasis than treating the water.

First, an excerpt written by a highly regarded wilderness physician:
“In recent years, frantic alarms about the perils of giardiasis have aroused exaggerated concern about this infestation. Government agencies, particularly the United States Park Service and the National Forest Service, have filtered hundreds of gallons of water from wilderness streams, found one or two organisms (far less than enough to be infective), and erected garish signs proclaiming the water ‘hazardous.”
And another, by researchers who surveyed the health departments in all 50 states and scanned the medical literature looking for evidence that giardiasis is a significant threat to outdoor folk:

Neither health department surveillance nor the medical literature supports the widely held perception that giardiasis is a significant risk to backpackers in the United States. In some respects, this situation resembles (the threat to beachgoers of a) shark attack: an extraordinarily rare event to which the public and press have seemingly devoted inappropriate attention.