There are times, however, when drinking water from a stream is a good idea in the short term. My father-in-law nearly collapsed from dehydration once on a hike because we had not carried enough to drink and, although the rest of us drank from a mountain stream, he didn't because he feared the infections you mention, jess. As we had arranged the walk, he and I descended the mountain back to the car while my husband and brother-in-law carried on over another peak to a spot where we would pick them up. If FiL had collapsed, it would have been a much greater emergency than an alimentary canal disorder later on. As it happens, he made it back to the car, but it was touch and go. He could hardly keep upright. I could not have shifted him if he'd fallen. Luckily there was a carton of orange juice in the car.
The rest of us were fine. Although I accept the risk you mention are there, jess, they are not enormous risks. Sometimes one has to balance risks in risky situations and choose the lesser one. Staying hydrated in mountain territory is worth taking a small risk for. And yes, it is a small risk, because most of the time, people who drink from mountain streams are fine.