I agree. The most common pathogens would take 3 to 5 days before the onset of symptoms, by which time you would hopefully be safe and have access to medical treatment. Also note that treatment does not necessarily remove or deactivate all organisms, it just reduces them to safe levels. So it seems the body has the ability to fight off infection, at least at lower levels of contamination.

I've often wondered if a person builds-up an immunity to these threats. When I was young I'd spend most of my free time in the woods, and thought little of drinking a few handfuls of crisp, clear water from a small fast-flowing creek or stream. I don't remember ever getting sick from it.