This article is about the infectious particles known as prions; for the bird, see Prion (bird).

A prion (IPA: audio) — short for proteinaceous infectious particle — is a unique type of infectious agent, as it is made only of protein. Prions are abnormally-structured forms of a host protein, which are able to convert normal molecules of the protein into the abnormal structure. Unlike other pathogens, prions cannot be denatured by protease, heat, radiation, and formalin treatments. Though the exact mechanisms of their actions and propagation are unknown, it is now commonly accepted that prions are responsible for a number of previously known but little-understood diseases classified as transmissible spongiform encephalopathies (TSEs). These include scrapie (a disease of sheep), Creutzfeldt-Jakob disease (CJD), and bovine spongiform encephalopathy (BSE or mad cow disease). These diseases affect the structure of brain tissue and all are fatal and untreatable.

Proteins showing prion behaviour are also found in some fungi. Some fungal prions may not be associated with any disease state and may have an evolutionary advantage for their hosts. So far all prions discovered are believed to infect and propagate by formation of an amyloid fold. However, since any infectious protein particle would be defined as a prion, other mechanisms may be possible.