Mad Cow Disease is a neurodegenerative disease , that effects the brain and spinal cord. The disease can be transmitted to human being who eat the brain or spinal cord. It is also spread between animals in the same way. The infection started because the brain , and other unwanted tissue was unwanted for human consumption but high in protien , so it was fed to cows as a cheap source of protein. Basically the milk is fine , because the disease doesn't target it.