I always use a water filter when dealing with wild water (i.e. not from a spigot in a campground). I can never be sure what is upstream from me or what has washed into the stream. When I was 12 I had severe runs for 4 weeks straight. Under those conditions one should be in the hospital, but we did not have insurance. It was so bad I couldn't go to school for 4 weeks, since I spent most of my time in the bathroom. I'm not sure if it was food poisoning or something else (it started 2 days after an overnight camping trip to the local river).

It was so bad I had to get a new buttocks, since the old one had a crack in it.

I never ever want to repeat that experience.