I ran out of water on a ten day bacpacking trip through the Sierras about 35 years ago and drank from a clear pool. This was in the days before filters were invented and you had to depend on tablets to purify the water. I didn't know about Giardia Lamblia until that trip and, to this day, I'd have rather not made his acquaintance. It's like a professional boxer using your stomach as a punching bag -- 24 hours a day!
