I think this was a news story last year, but now I can't remember if I made it up. (Does that ever happen to you?)
This fat lady was feeling really ill and saw that she had some sort of growth under her arm, so she went in to the doctor. He began to examine her, and when he lifted up one of her rolls of fat he found a whole tuna sandwich buried inside, rotting and covered with fungus.
Real or not, that's probably one of the most disgusting stories I've ever heard. I know it's very junior high-ish, but what's the grossest story you know?