“Most scientists think that BSE is caused by a protein called a prion. For reasons that are not completely understood, the normal prion protein changes into an abnormal prion protein that is harmful. 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. A common sign of BSE in cows is incoordination. A sick cow has trouble walking and getting up. A sick cow may also act very nervous or violent, which is why BSE is often called “mad cow disease.”

U.S. Food & Drug Administration