I think the Rockwell article linked above, or possibly another source, mentions that more than 7% of everybody in the U.S. is infected with Giardia at any one time, although most are asymptomatic.

Apparently, if you shake hands or share finger food with any of them, at umm...er, the wrong moment....you too can join the club, but you probably won't get sick and you might gain immunity.

I read a survey that said in a given month, 5% to 10% of everybody in America gets the trots.....Giardiasis is extremely common, and we're absolutely not talking just among campers. But apparently, various viruses (Giardia is protozoa) are even more commonly the culprit..

Vectors for all the bugs are the same:..mostly hand-to-mouth, or contaminated food...which is pretty much the same thing.....water-borne is the other possibility, but comparatively rare.....

Merely in terms of statistical chances, sharing GORP with the wrong person.......is a far greater hazard than tainted water..

This is the general point that Rockwell makes, although most of the (voluminous) research he cites pertains specifically to the Sierra Nevada where he notes Giardia, in infective amounts, is nearly unrecorded despite lots of testing.

All of this, by the way, isn't a matter of Rockwell's opinion. He documents it all rather carefully, and many of his citations can be viewed on the Web.