That's a good point - it can't be the sole cause, but ingesting the BT toxin, which is designed to break open the stomach doesn't help.
I've read that bile duct congestion, causing any kind of indigestion and the build up of material in the digestive system will cause parasites, which can also permeate the walls, as well as casein in milk, grains, alcohol, etc.
You've most likely heard of all of these ... it's a very odd phenomena anyways