Protein frags. Prions are very small proteins, often described as protein fragments because they are too small to have once been thought as having any bio function. So, like proteins, they are strings of amino acids. High concentrations of certain abnormal prions can replicate themselves and cause illness (as colleagues have described). But normal prions are found throughout the body.
Prion. Prions are proteins that have changed physical conformation, so that the normal function of the protein inside the cells cannot be accomplished. The prions' abnormal folding 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.