Prions (short for proteinaceous infectious particle) are infectious protein structures that replicate through conversion of normal host proteins of the same type. Though the exact mechanisms of  their actions and reproduction are unknown, it is now commonly accepted that prions are responsible for a number of previously known but little-understood diseases generally classified under transmissible spongiform encephalopathy diseases (TSEs), including 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. 

Not all prions are dangerous; in fact, prion-like proteins are found naturally in many (perhaps all) plants and animals. Because of this, scientists reasoned that such proteins could give some sort of evolutionary advantage to their host.