It goes alongside "if you sleep with your belly exposed you'll get a cold" superstitions. We here in the west aren't much better. "If you stay out in the cold/rain, you'll get sick." We say "God bless you" when someone sneezes because people thought sneezing was the soul trying to leave the body. And, of course, if you go back far enough, you could always use a good bleeding with leeches to cast the devil out of an ill person.
Dr_Z said: I don't even think it was an old quack cure I think it was Miku Hatsune that started the leek up the ass stuff. But Patchouli sure did recover fast from the fear of a leek/negi raping.
I wonder if she is constantly faking a cold to get leeks up the poopchute.