I drank directly from mountain streams for decades before I ever purchased or used a water filter. During that time I personally had one bout of illness I directly attributed to bad water, in Rocky Mt. National Park, when I was about 17 years old. It was a miserable experience, but I survived. My brother and his wife also fell prey to giardia once. They also survived, but it was not a happy experience either.

These days I will still drink directly from a mountain water source, even though I carry a water filter, but only when I can see that the water is emerging from a clear, cold and copious spring a short distance from where I collect the water. I regard filtering clear, fast running water to be mostly an overabundance of caution, but it isn't entirely a baseless superstition.