Mad cow disease, also known as bovine spongiform encephalopathy (BSE), is a progressive neurological disorder of cattle that results from infection by an unusual transmissible agent called a prion. Most scientists believe that BSE develops when a normal cellular protein in the brain known as prion protein loses its normal shape and triggers a chain reaction of further prion-protein misfolding, causing the destruction of brain cells and nervous system tissue. The prion protein changes into an abnormal prion protein that is harmful, and the body of a sick cow does not even know the abnormal prion is there. Without knowing it is there, the cow’s body cannot fight off the disease. Eating infected cattle products, including beef, can cause a human to develop variant Creutzfeldt-Jakob disease (vCJD), a human version of mad cow disease. The infectious agent that causes mad cow disease is an abnormal version of a protein normally found on cell surfaces, called a prion. For this reason, the greatest risk of acquiring vCJD is associated with ingesting nervous system tissues, such as the brain, spinal cord, and eyes, from infected cattle. Milk and milk products are not believed to transmit the mad cow agent.