“Mad cow disease” is the common name for bovine spongiform encephalopathy, or BSE, a disease that slowly destroys the brains of cattle. It’s an especially worrisome disease because the agent of infection is a nonliving protein particle that cannot be destroyed by cooking, and that appears to cause a similar and fatal disease in people who eat infected beef. We still have a lot to learn about it.
BSE originated in the early 1980s when cattle were fed by-products from sheep suffering from a brain disease called scrapie, whose cause appears to be a chemically stable protein aggregate called a prion. The sheep prions somehow adapted to their new host and began to cause brain disease in the cattle.