It isn't as if a fair number of scientists interested in public health haven't tried to investigate this subject. It's just that they can't find anything to study.

If you look into the various scientific literature on the Web, you'll see that backcountry water contamination in the USA has never been proven as a health hazard, despite some rather impressive scientific efforts.

So that leaves folklore.

That said, there's some spooky water out there. I've drank it mostly unfiltered, and sometimes filtered. The older I get, the more I lean toward filters when it looks spooky. In western alpine mountains in Washington and Canada, I'm happy to drink untreated water.

My two most dubious experiences:

Five years ago I bought my first filter at behest of girl friend, and camped on a small stream forty miles from NYC. The filtered water didn't taste too good. Next day, I walked upstream and found small sewage treatment plant a mile away (operated by Harriman State Park near Queensboro Lake.)

In early 70s, I camped for a week with a group on upper Kennebec River in Maine. There was a sparse, local population that mainly drew its drinking water directly from river and had done so for generations. They probably also used "straight pipes" for plumbing. We drank the river water without treatment.

One of our group came down with hepatitis. I'm quite skeptical that it came from the river water. It's certainly possible though. He got sick in my cooking pot, which I kept until fairly recently.

We all got shots. I'm fine, and didn't treat any water until decades later.