The first time I was bitten by a leech, I was swimming in a West Virginia river. I remember the scene perfectly: I crawled out to dry off and tried to wipe the mud off my knees, but the leech wouldn’t ...

You might think you know leeches: Go for a swim in the wrong shallow lake, and you’ll emerge covered in sleek black bloodsuckers that have decided you’re their next meal. But inside a leech, ...