How does a prion harm an organism?

Prion: Prions are proteins that have changed conformation, so that the normal function of the protein inside the cells won't work. The prions' also causes other normal proteins to misfold and make more prions, spreading the process. The prions affect many cells in the body, but they particularly harm nerve cells in the brain, causing progressive rapid degeneration. They are not alive, and have no dna.

The abnormal folding: That occurs in prion diseases in animals produces clumping of these proteins forming plaques. It is likely that the deposition of such plaques damages nerve cells but it is also possible that decreasing amounts of the normal functioning has a role. At least in some yeasts, and probably in other organisms, prions can have non-damaging roles in the cell.