Wait, wait, wait there Dr. Science. Following up on the references, I think the problem is a bit overstated. I was worried about your statement:
I was able to find only two studies that tested hikers BEFORE and AFTER a trip to the field. They showed a minimum of 5.7% of hikers contracted Giardia in a single trip. Seems high, but it does show the risk can be very high at times. http://www.ncbi.nlm.nih.gov/pubmed/937629
It is high, but the reference is one specific incident in Utah where the authors couldn't figure out the source. As I said (and purely anecdotal yet worth something...) the Sierra, even in cattle areas, just doesn't get many cases of giardia.
His calculations on how many liters of water one would have to drink to get giardia in the Sierras are worse than useless. For one he uses water tests decades old. And he uses 10 cysts as the "infective dose." You will actually find the "10 cysts" repeated many places on the internet, but again, it's untrue. Experts have determined that there is a 2% chance of being infected
by a single cyst. http://www.waterbornepathogens.org/inde
... &Itemid=38 The FDA says Ingestion of one or more cysts may cause disease. [Emphasis added]
Following up with that source you find that 2%/single cyst stuff is extrapolated and refers to infection (carrier) not symptoms:
Thus, although the individuals were infected, they did not have classical giardiasis.
Derlet (a friend) has done extensive testing for years. However, he's done very little testing for giardia. It's almost entirely e coli which he uses as an indicator of fecal contamination from all sources. There's a definite increasing correlation in e coli between pristine (no human or stock use), human use only and stock & human use.
There, bacterial contamination was easily high enough to sicken hikers with Giardia, E. coli and other diseases
That's what I indicated earlier, but even in yuckky cow areas, the symptomatic cases of either giardia or even intestinal grunge is not a major problem (there's definitely cases and some years they're significant). Derlet has not studied symptomatic outbreaks of disease in those areas. For sure the potential seems to be there but (and I'm very open to correction) the clinical cases aren't.
Whew. Maybe everyone else is right, this does get kinda whacked to death. But I guess my main point is not overstating or exaggerating the risk of giardia. We're slowly coming back to a reality-based attitude where people don't go thirsty because they've not treated the water or their pump broke or whatever. As you say, it's a personal choice, but should be one based on a realistic assessment of actual risk which, I maintain, is very low.