I don't treat even in the Whitney Zone. At Trail Camp, I always make my to the Trail Camp pond water inlet, where clean fresh water is always flowing clear.

Here's another Bob R contribution: 60,000 Liters of Water Consumed -- Untreated

In addition to hygiene, am pretty sure food poisoning can be blamed for quite a few trail illnesses. On a JMT trip, I met two people affected by serious intestinal illness. One was immobilized for several days, and he said the problem was due to letting his pack food get too warm (sausage I believe). Met another fellow whose partner had to bail and was in the hospital due to food poisoning and its after effects. Wagga once went on a Sierra Club National trip, and most hikers got sick -- they blamed a package of fresh bean sprouts. So often, people will blame the water, without even thinking about the more obvious sources.