Prions are infectious proteins that have become misfolded and cause transmissible spongiform encephalopathies in mammals by converting normal prion proteins into additional misfolded prions. These prion diseases affect the brain or nervous system, are currently untreatable, and always fatal, such as mad cow disease in cattle and Creutzfeldt-Jakob disease in humans.

3. Prion
• A prion is an infectious agent, specifically a protein in
a misfolded form. The word prion, coined in 1982 by
Stanley B. Prusiner, is derived from the words protein
and infection. (can be referred to as the prion protein
(PrP)).
• All known prion diseases in mammals affect the
structure of the brain or other neural tissue and all
are currently untreatable and universally fatal.
• Prions are responsible for mammalian transmissible
spongiform encephalopathies, including bovine
spongiform encephalopathy (BSE, also known as
"mad cow disease") and scrapie in sheep. In humans,
prions cause Creutzfeldt-Jakob Disease (CJD).