I used to backpack with an internist in the 70s and 80s. This was when reports of water-borne diseases were increasing in Idaho's wilderness and water filters were becoming expensive but necessary accessories. This doctor pointed out maintaining a sterile water source is far more complicated than anyone believes. If you rinse your hands in the lake, walk across a creek and then touch your boots or dip your bandana into the stream and wipe your face, there's no need to filter your water except to remove debris. At the microscopic level, you've totally compromised your clean water regimen.
I think this is a version of the "one germ will kill you" fallacy. OK, it's just barely possible -- but Schrodinger's cat may have slept here, too.
However, a normal person's immune system can handle some number of most etiologic agents. One flu virus? your immune system will probably contain it before you come down with the flu. One giarda cluster? ditto. The problem comes when you ingest 100 giardia clusters, or a few thousand flu viruses. Then they can reproduce fast enough to overwhelm your immune system, and you'll get sick.
The question is, how many is too many? and how do you keep the number you're exposed to below that "too many" number? The answer is not always clear, but it's part of why you wash your hands before eating, rinse off fruits and vegetables before preparation, and clean out your hydration pack. It's almost impossible to sterilize your hands, and not many of us have an autoclave to cook the pack before or after each outing, but we do what we can to get the population of germs down.
