Great question, Kent..it's not really known.  In fact, prion is really a theory, at this point.  The prion protein itself resides in the tissue normally (be it brain, spinal cord etc..) and is transformed to an "abnormal" configuration by an unknown agent (yes, possibly virus..or other vector).  The protein goes from an orderly "alpha" configuration (spiral or chain), into a beta pleated sheet structure.   I have not read of any evidence of these prions being isolated however..so I am unclear of how the theory got started in the first place.  I'd have to go back and read Pruisner's work to have a better idea.

BTW...and this is an aside, but a friend of mine "worked" with Pruisner for a year.  He was considering a career in research, after we had completed our residencies together in Neurology.  He got NO guidance from Pruisner, and was left on his own in the lab.  Bewildered, and frustrated from this experience he left after one year, and eventually migrated into clinical practice.  Pity really...he would have been a very gifted scientist.  He had some choice words to say about Dr.P...but I won't repeat them here.

Gene