As I said it causes a certain kind of brain protein to get folded wrong. This causes plaques and holes to form in the brain eventually getting in the way of normal brain functioning and causeing demetia (like Alzeihmers) and eventually the patient dies. It generally takes a long time to happen but is very sad and tragic of course. Mad cow disease is actually called BSE or bovine spongioform encephalitis, which basically means it makes cow brains look like sponge in science terms.
~~Nonpathogenic microbes are our friends~~