Prions (proteinaceous infectious particles) are small, infectious protein particles that cause transmissible spongiform encephalopathies (TSE), incurable diseases that primarily attack the nervous system by creating amyloid plaques. The name prions was introduced by Stanley Prusiner, a scientist who in 1982 proposed the first hypothesis about prions. The prion theory is based on the discovery of an infection-associated low molecular weight protein in TSE tissues. Several lines of evidence suggest that glycosylated protease-resistant protein (PrP) is a component of the infectious particle. \(PrP^{Sc}\) (from scrapie), a tissue-destroying protein, has 30% α-helix and 43% β-pleated sheet, in contrast to the cellular \(PrP^{C}\) (from cellular), which has 43% α-helix. Prions are thought to convert normal proteins into infectious agents and to convert healthy molecules into diseased ones. Studies on the model organism Saccharomyces cerevisiae describe prions as evolutionary factors in yeast cells and provide evidence for the influence of prions on the viability of individuals under unfavorable conditions. This paper describes the structure and action of prions and prion diseases in animals and humans.