A prion (IPA: listen ) — short for proteinaceous infectious particle (-on by analogy to virion) — is a type of infectious agent composed only of protein. They cause a number of diseases in a variety of animals, including BSE in cattle and CJD in humans. All known prion diseases affect the structure of the brain or other neural tissue, and all are untreatable and fatal.citation needed]
Prions are believed to infect and propagate by refolding abnormally into a structure which is able to convert normal molecules of the protein into the abnormally structured form. However, the term in itself does not preclude other mechanisms of transmission. All known prions induce the formation of an amyloid fold, in which the protein polymerizes into a fiber with a core consisting of tightly packed beta sheets. Other mechanisms may exist in yet undiscovered infectious protein particles. This altered structure renders them quite resistant to denaturation by chemical and physical agents, although infectivity can be reduced by these treatments, making disposal and containment of these particles difficult.
Proteins showing prion behaviour are also found in some fungi. Some fungal prions may not be associated with any disease; it is unknown whether these prions represent an evolutionary advantage for their hosts.