Iodine for water purification, on a cross country horse ride a couple of years ago, we had to drink from some very dodgy water supplies, I used iodine to clean my water,(the same way my mum did when I was small), and was the only one of about 35 people who didn't get a bout of gardia when we got home. I was tested because it is a notifiable disease in Aus, and didn't have any sign of it.
Others on the trip boiled their water or used chlorine tablets and still got it.