The term "prion" is derived from proteinaceous infectious particles and refers to the pathogen that causes transmissible spongiform encephalopathies (TSEs).
This small infectious particle is a disease-causing form of a protein called a cellular prion protein (PrPc).
Prion protein (PrPc) is mainly found on the surface of cells in the central nervous system (CNS) but it is also located in other bodily tissues.
A prion is composed of an abnormally folded protein that causes progressive neurodegenerative conditions with two of the most notable being Bovine spongiform encephalopathy (BSE or mad cow disease) which can be seen in cattle and livestock and Creutzfeldt-Jakob disease (CJD) seen in humans.