Some diseases scientists once referred to as "slow viral infections" such as scrapie in sheep, chronic wasting disease in deer, bovine spongiform encephalopathy ("mad cow" disease) and Creutzfeldt-Jacob disease in humans are actually the result of very small protenaceous infective particles (prions).

In 1982, Stanley Prusser discovered that a very simple prion protein (PrP) seemed to be responsible for diseases such as those mentioned above.  He recognized that there were two stable forms of PrP, one composed of alpha helices (a, above) found in nerve cells and another having beta helices (b, above) found only in diseased mammals.  He suspected that the mutated PrP could overwhelm healthy PrP, leading to cell death and the development of large vacuoles in brain tissue as found in spongiform encephalopathy.