"Nobody" it is and so it shall remain due (among other difficulties) to the lack of control groups.

I recall reading about an individual who studied AT hiker illnesses (mostly GI related, iirc). He may have published it somewhere. But I doubt it's of any use either.

Personally speaking .... i've never been very concerned w/giardia, etc., while hiking in the USofA. But i'm pretty careful in comparison to most hikers I know.

One other consideration: Contaminated water sources. E.g. on a recent hike I knew that one of the larger creeks was said (by reputable sources) to be contaminated w/Arsenic. Lotsa mining in the area in past years. Surprisingly enough, when discussing water sources w/a highly experienced group leader (they'll remain anonymous), she stated that since the creek was running at high volume it would be fine to drink from. I didn't contradict her. But when I reached the creek .... I moved on. ;-)

Peace,

Richard.