The similarities of certain animal and human TSE was noted beginning in the 1950s. Research published in the late 1960s demonstrated that the agent that causes the sheep TSE called scrapie was extremely resistant to deactivation by ultraviolet (UV) and ionizing radiation2, treatments that would destroy anything with nucleic acid. However, the nature of the infectious agent remained a mystery and suggestions included small DNA viruses, membrane fragments, polysaccharides and proteins. Some scientists theorized that scrapie was caused by an agent that did not depend on nucleic acid for its ability to reproduce.3,4

In 1982, Stanley Prusiner, M.D., of the University of California-San Francisco announced in the journal Science5 that he had succeeded in purifying the scrapie disease-causing agent and had identified it as a protein. In the journal article, Prusiner noted that “because the novel properties of the scrapie agent distinguish it from viruses, plasmids, and viroids, a new term "prion" was proposed to denote a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids.” In 1997, Prusiner was awarded the Nobel Prize for his discovery.

While the prion theory dominates the science of TSE, other theories exist about the cause. These postulate that the real infectious agent does have DNA and includes a theory that a slow acting virus is the cause based on finding tiny virus-like particles in neural tissue associated with TSE. Another theory is that TSE are caused by virinos — hypothetical particles consisting of nucleic acid in a protective coat of host cell proteins. A bacterial theory proposes that a tiny, wall-less bacteria called spiroplasma attaches to normal prion protein in the brain and causes it to misfold. 

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