The responsible parasite is Fasciolopsis buskii, the human intestinal fluke, a flatworm.
It is a human parasite although it can also parasitize other species. It normally lives quietly in the intestine. (The goal for all larger parasites is to live quietly. After all, your demise is their demise.) They were meant to pass their thousands of eggs with your bowel movement, outside, to some pond where snails live. But when the eggs hatch before THE CURE FOR ALL DISEASES 332 they leave your body and are allowed to continue their development inside you, the setting is right for cancer to develop.