What is mad cow disease, viral or bacterial?

Prion: This is an abnormal protein, neither a virus nor a bacterium. It is an altered version of a normal brain protein that catalyzes the transformation of this protein into copies of itself.

Slow virus: It's viral. It is a type of infection in which infection may not be evident for decades. Slow viruses are notoriously difficult to culture but advanced dna techniques and electron microscopy can prove their presence.