Swimming in Minnesota lakes: always check for leeches. It seems like they hang out close to shore, so swimming near the dock or weeds increases the chance of being an involuntary blood donor.

New Guinea: they have land leeches that hang out at the end of branches, like ticks. Hikers use anti-leech sleeves and gaiters.

I say: nuke from orbit, it's the only way to be sure. ;-)