The disease of mad cows is due to prions. Prions use certain intestinal cells to enter the body, but most do not do it alone, but they do it by grabbing another protein, ferritin.
Researchers from the American city of Cleveland have followed in the laboratory the way of prions in the body. A piece of meat with prions reproduces the alterations suffered in the digestive system, adding oral, stomach and intestinal enzymes. Unlike other proteins, prions remain whole after the attack of enzymes, as well as ferritin. Marking prions and ferritin with fluorescence, they have followed their path and have seen that they internalize each other through intestinal cells.
It seems that this association has allowed the passage of prions of some species to others. Ferritin is a protein very similar in many animal species.