Scientists have stumbled upon a mystery. Diseases known as Transmissible Spongiform Encephalopathies (TSEs), which include the well-known mad cow disease or Bovine Spongiform Encephalopathy (BSE) that is so much in the news these days, are caused by something that so far has eluded scientists. To date, we understand that diseases are caused by infectious agents: viruses, bacteria, fungi, and parasites. Scientists have been unable to detect any of these organisms in the case of these TSEs. One theory that has become fairly widely accepted is that a protein called a prion (PrP) is responsible for these diseases.