Prions are proteins that are infectious. Indeed, the name prion is derived from "proteinaceous infectious particles." The discovery of prions and confirmation of their infectious nature overturned a central dogma that infections were caused by intact organisms, particularly microorganisms such as bacteria, fungi, parasites, or viruses. Since prions lack genetic material, the prevailing attitude was that a protein could not cause disease.

Prions were discovered and their role in brain degeneration was proposed by Stanley Pruisner. This work earned him the 1997 Nobel Prize in medicine or physiology.

In contrast to infectious agents that are not normal residents of a host, prion proteins are a normal constituent of brain tissue in humans and in all mammals studied thus far. The prion normally is a constituent of the membrane that surrounds the cells. The protein is also designated PrP (for proteinaceous infectious particle). PrP is a small protein, being only...