A prion, pictured above, is an infectious agent comprised only of a protein in misfolded form. It lies in stark contrast to all other known infectious agents; even the simplest bacterium or virus contains nucleic acids (either DNA, RNA, or both) but amazingly, prions seem to have neither. Instead, they propagate by transmitting a misfolded protein state - meaning that when a prion enters a healthy organism, it induces existing, properly folded proteins to convert into the disease-associated, prion form. The prion essentially acts as a template to guide the misfolding of more cellular proteins; newly formed prions can then go on to convert more other proteins, triggering a chain reaction that produces the prion form in exponentially large numbers.
Prions are responsible for the transmissible spongiform encephalopathies in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as “mad cow disease”) in cattle and Creutzfeldt–Jakob disease (CJD) in humans. All known prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and universally fatal.