You didn't say what lake or even region you were considering drinking from but knowing nothing about springs which are supposed to be safe to drink from I've also considered and experimented with drinking directly from Sierra lakes such as Smith Lake and Gilmore Lake in Desolation Wilderness without any ill effects. You might have gotten giardia, or it might be all corporate advertising hype to sell water filters, I don't know, but my anecdotal experience makes me wonder.
I also drank the water in the Philippines and only a couple time did I get a little diarrhea which Imodium took care of immediately. I had had Typhoid and HepA/HepB immunizations so I wasn't being completely reckless.
