Prion replication in cells
Prions are reconfigurations of proteins that are naturally produced by cells. When the prion version meets the normal version, it causes the normal version to flip and become the prion version. This cascade effect increases the number of prions in a way analogous to the spread of an infection. Eventually this build up of novel protein (which does not degrade) leads to the prion diseases (including BSE or Bovine Spongiform Encephalopathy, or Mad Cow Disease).