Be Careful Where You Swim This Summer

That glistening swimming hole might look—and feel—refreshing on a sweltering day. But writhing in pain from stomach cramps, nausea, and vomiting? Staring at the bathroom ceiling instead of the blue, sunny sky? Not so much.

Jumping into even the prettiest and clearest rivers, lakes, creeks, and other natural bodies of water can expose you to a cesspool of unpleasant and invisible fellow swimmers—most commonly bacteria, viruses, and parasites. “These germs are microscopic, so you’re not...