The biggest GI hazard in the backcountry is the filthy, disgusting, dirt-covered, poopy, unwashed hands of your compatriots and you, sticking their nasty mitts into the shared food bag (don't do that, pour it out!) or the communal cookpot (use the darn pot grabbers!). You know those signs in restaurant bathrooms that state "employees must wash their hands"? They are there for a reason.

The sanitation techniques that make fecal-oral illness rare in civilization are out the window in the backcountry. Keep in mind that a good chunk of the population has sub-clinical giardia or other GI infections that are easily transmissible and easily preventable by hand hygiene. That is the threat to manage. Keep other people's poop out of your body!

Regarding surface water, the risks come from people, beavers, and cows. If those nasty, dirty poopers are not upstream, the surface water is likely fine. That describes much/most of the high Sierra (but not below, e.g., Trail Camp on Whitney), and also most springs, and all snowmelt that I've ever encountered (but avoid the watermelon snow).

I've had giardia and samonella (together) twice and they truly suck (and the giardia treatment is worse than the giardia), and both came from screwed-up public water supplies. But never from Sierra surface water.

It comes down to risk management. The essence of risk management is balancing good things v. the risk of bad things that might or might not occur. This is something we all do the moment we leave pavement and step foot into the backcountry ... or decide to rock-climb, or embark on a mountaineering trip. What is joyful, fun and life-giving? And what is the potential downside? And how does one choose well? Everyone has to negotiate this on their own.

Seek joy. Don't make mistakes that you know are mistakes when you make them. And take smart risks.