"But I will happily treat all backcountry water for the rest of my life to avoid the next case."
Yes, I certainly feel the same way. I have had it, and it has gone away. Not really dibilitating, but, I didn't want to be far from a bathroom from three days. This was long ago...well over 50 years...
The point is that a large segment of the population is asymptomatic or only mildly effected. Do you know where you stand? Once you have had it, what are the chances of reinfection? What of the rural population that drink from untreated wells every day? Spring boxes can be real bad, yet, these people never get sick. Aquired immunity?
Yes, side waters are safest. This is where I sometimes drink. But, there was no paths and few animals in the area. Cold, clear, spring fed, running water, no different that what I have been drinking, probably a whole lot better.
That said, do I recommend treating water? Treatment is always recommended. Most hikers live in urban environments. Water is typically piped into their homes from some common source and it is treated. Chances are, they are highly suseptable to gardia. For an old farm boy, I can get away with drinking large amounts of gardia in my water. I am not immune, but, there is a level of tollerance. How much, I don't know. What is the mechanism? Why does it not seem to bother me as much as others? No answers, sorry. Just these and more questions....