A proteinaceous infectious particle which causes one of a number of non-inflammatory, slowly-developing degenerative neurological diseases which are characterised by extracellular plaques, notably bovine spongiform encephalopathy (mad cow disease), scrapie in sheep and Creuzfeld-Jakob disease in humans. Unlike other infections agents, it is apparently composed entirely of a single protein, PrP^Sc, which differs from the normal cellular protein, PrP^C, only in its conformation; the benign conformation is converted into the infectious conformation when the two come together, by a rare spontaneous conversion or by a mutation of PrP which allows it to more easily assume the infectious conformation.