The phrase "mad cow" refers to a disease called Bovine Spongiform Encephalopathy (BSE) that affects cows. It makes them act strange, like being very aggressive, unable to walk properly or behaving abnormally. Full definition
It could, however, potentially carry other pathogens that might be transmitted to humans, such as the protein that causes mad cow disease. (scientificamerican.com)
The protein ultimately responsible for mad cow disease might be necessary for healthy brain function. (wi.mit.edu)
Would you want to know if you were carrying human mad cow disease? (newscientist.com)