Swimming in a lake could lead to some kind of parasitic infection, Dr. Oz says. "The most common waterborne parasite is called giardia, and giardia actually afflicts about 3 million people a year."
Giardia irritates your intestinal tract—similar to the feeling of a stomach virus. To tell the difference, Dr. Oz says you can take a blood test for it or you can watch your poop. Since it affects your ability to absorb food correctly, if you have giardia your poop will float. "That's one of the clues we'll sometimes look for when you have an intestinal parasite. It will destroy the way that you actually digest food."
Dr. Oz says the problem is that giardia is found in a lot of water in the United States—especially water that hasn't been purified or filtered.