Giardia and Cryptosporidium aren’t going to give you a positive effect. They are going to make you sick. And those are the biggest risks from untreated stream water.

Any many other microorganisms including Leptospira. The latter can be acquired through the skin by swimming in the wrong areas in Hawaii [1]. Giardia can be difficult to treat and give you horrible problems for years as happened to one friend. Another friend acquired boils with visible nematodes under the skin from wading near Belem in Brazil. As a field biologist one is constantly reminded that nature is not benign. However local knowledge can usually keep one out of trouble, with the disclaimer that sadly in almost all of the world even its wildest most remote corners, the organism that poses the greatest danger is Homo sapiens.

1 http://health.hawaii.gov/about/files/2013/06/leptobrochure.p...

There is always the appeal to nature argument. Slightly off topic. On one of my hikes with my dog. I wouldn't let him drink from a stream. Some people near by waved to me and said "It is fine! Dogs know good water". I thought how absurd. Another time we came across some small animals. He has a high prey drive. He made some aggressive advancements towards the creatures. I called him off. A person sharing our hike made the comment that it was okay. If he ate the small animal it would be good for him. It is natural. I said no the possibility of my dog contracting a parasite like tape worm would be a very bad thing. Nature can kill or hurt you in very natural ways.