Un prion es un agente infeccioso formado por una proteína denominada prionica capaz de formar agregados moleculares aberrantes.Su forma intracelular puede no contener ácido nucléico. Produce las encefalopatías espongiformes transmisibles, que son un grupo de enfermedades neurológicas degenerativas tales como la tembladera, la enfermedad de Creutzfeldt-Jakob y la encefalopatía espongiforme bovina. La proteína del prion es una sialoproteína patógena, la cual tiene alterada su estructura secundaria, teniendo un incorrecto plegamiento de su estructura terciaria. A diferencia del resto de los agentes infecciosos (virus, bacterias, hongos etc...), que contienen ácidos nucleicos (ya sea ADN, el ARN, o ambos), un prion solamente está compuesto por aminoácidos y no presenta material genético. El prion, palabra acuñada en 1982 por Stanley B. Prusiner al investigar una serie de enfermedades de carácter crónico e irreversibles que afectaban al sistema nervioso central, es un acrónimo inglés derivado de las palabras proteína e infección. Los priones son los responsables de las encefalopatías espongiformes transmisibles en una variedad de mamíferos, incluida la encefalopatía espongiforme bovina (EEB, también conocida como "enfermedad de las vacas locas") en el ganado y la enfermedad de Creutzfeldt-Jakob (ECJ) en humanos. Dichas proteínas mutadas forman agregados supramoleculares y son patógenas con plegamientos anómalos ricos en láminas beta, y autorreproducibles. Los priones se propagan mediante la transmisión de proteínas anómalas con mal plegamiento. Cuando un prion entra en un organismo sano, actúa sobre la forma normal del mismo tipo de proteína existente en el organismo, modificándola y convirtiéndola en prion. Estos priones recién formados pueden pasar a convertir más proteínas, provocando una reacción en cadena que produce grandes cantidades de la proteína prion. Todos los priones conocidos inducen la formación de amiloides plegado, en los que actúan polimerasas formando un agregado que consiste en apretadas hojas ß. El período de incubación de las enfermedades prionicas se determina por la tasa de crecimiento exponencial asociados con la replicación de priones, que es un equilibrio entre el crecimiento lineal y la rotura de los agregados (hay que tener en cuenta que la propagación del prion depende de la presencia de la proteína normalmente plegada en la que los priones pueden inducir plegamiento. Los organismos que no expresan la forma normal de la proteína prionica no pueden desarrollar o transmitir la enfermedad).

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A prion is an infectious agent composed entirely of protein material, called PrP (short for prion protein), that can fold in multiple, structurally distinct ways, at least one of which is transmissible to other prion proteins, leading to disease that is similar to viral infection. They are suspected to be the cause of transmissible spongiform encephalopathies (TSEs) among other diseases. Prions were initially identified as the causative agent in animal TSEs such as bovine spongiform encephalopathy (BSE)—known popularly as "mad cow disease"—and scrapie in sheep. Human prion diseases include Creutzfeldt–Jakob disease (CJD) and its variant (vCJD), Gerstmann–Sträussler–Scheinker syndrome, fatal familial insomnia, and kuru. A 2015 study concluded that multiple system atrophy (MSA), a rare human neurodegenerative disease, is caused by a misfolded version of a protein called alpha-synuclein, and is therefore also classifiable as a prion disease. Several yeast proteins have been identified as having prionogenic properties as well. A protein as a standalone infectious agent stands in contrast to all other known infectious agents such as viruses, bacteria, fungi, and parasites, all of which contain nucleic acids (DNA, RNA, or both). For this reason, a minority of researchers still consider the prion/TSE hypothesis unproven. All known prion diseases in mammals affect the structure of the brain or other neural tissue; all are currently untreatable and universally fatal. Prions may propagate by transmitting their misfolded protein state: When a prion enters a healthy organism, it induces existing, properly folded proteins to convert into the misfolded prion form. In this way, the prion acts as a template to guide the misfolding of more proteins into prion form. In yeast, this refolding is assisted by chaperone proteins such as Hsp104p. These refolded prions can then go on to convert more proteins themselves, leading to a chain reaction resulting in large amounts of the prion form. All known prions induce the formation of an amyloid fold, in which the protein polymerises into an aggregate consisting of tightly packed beta sheets. Amyloid aggregates are fibrils, growing at their ends, and replicate when breakage causes two growing ends to become four growing ends. The incubation period of prion diseases is determined by the exponential growth rate associated with prion replication, which is a balance between the linear growth and the breakage of aggregates. The propagation of the prion depends on the presence of normally folded protein in which the prion can induce misfolding; animals that do not express the normal form of the prion protein can neither develop nor transmit the disease. Prion aggregates are extremely stable and accumulate in infected tissue, causing tissue damage and cell death. This structural stability means that prions are resistant to denaturation by chemical and physical agents, making disposal and containment of these particles difficult. Prion structure varies slightly between species, but nonetheless prion replication is subject to occasional epimutation and natural selection just like other forms of replication.

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