Alternate diagnosis: it's a parasitic worm. What happens is that you eat contaminated food & pass the eggs into your digestive system. They pass through your stomach undamaged & hatch in your small intestine. The little critters (some form of parasitic worm) starts wiggling around & growing. A week or two later, it emerges from the anus (not while shitting - it crawls out, generally while you're inactive) and burrows into the nearby flesh where it forms a cyst-like bump. At that point, it does some sort of metamorphosis or whatever the hell it's called where it changes from one form to another (like a caterpillar to a butterfly.) At that point, it becomes much more painful. Due to increased touching of that area, you end up contaminating your hands, and ultimately contaminate other food (or maybe not - the person you caught it from certainly contaminated your food though.)
I can't remember the name of the parasite, but it's closely related to the guinea worm.
http://en.wikipedia.org/wiki/Dracunculiasis