Prions are defined as "infectious proteinaceous agents" and there are
those that maintain that NO nucleic acid is meaningfully associated with the
Prion protein. If this is true, then their infectiousness and capacity to
cause disease might be likened to the bad apple in the barrel of good ones;
although the barrel always contains the same number of apples, eventually due to the one bad apple, all of them go bad. The information for making the Prion
in the first place lies in a cellular gene which, presumably, performs some
necessary (if unknown at this time) function for the host organism. If this
gene mutates, or conceivably if something happens to the normal Prion protein
turning it into the proverbial bad apple, then disease progression may begin.
Although unproven, it remains possible that the agent of BSE is an altered
form of the scapie Prion of sheep.
This short answer in response to recent query by John Brooks.