Prions are, amino-acid-for-amino-acid, identical to a nerve tissue protein called PrPC. However, they're folded in a radically different manner, such that instead of performing the normal duties that a PrPCmolecule would, they instead act as catalysts to re-fold PrPC into new prions.Thus, one prion will, when introduced into neuronic tissue (such as brain tissue), create more prions, and these will in turn create even more prions -- all the while robbing the tissue of the PrPC it needs to function properly.