What type of virus causes mad cow disease?

Mad cow disease, also known as bovine spongiform encephalopathy (BSE), is caused by a prion, not a virus.

Prions are misfolded proteins that can cause other proteins to misfold, leading to the formation of aggregates that damage brain tissue. These aggregates are resistant to normal breakdown processes in the body.

It's important to note that prions are not viruses. Viruses are composed of both protein and genetic material (DNA or RNA), while prions are only composed of protein.