prion

n. theoretical infectious agent that is composed only of a protein (and has no genetic material)

A prion () — short for proteinaceous infectious particle (-on by analogy to virion) — is an infectious agent composed only of protein. They cause a number of diseases in a variety of animals, including BSE in cattle and CJD in humans. All known prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and fatal. Mice genetically modified to avoid the symptoms are important models of study. In general usage, prion can refer to both the theoretical unit of infection or the specific protein (e.g., PrP) that is thought to be the infective agent, whether or not it is in an infective state. Prion diseases can result from modification of a host-encoded glycoprotein (i.e. PrP)("protease resistant protein") which disrupts normal synaptic function.
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Prion /pri·on/ any of several transmissible forms of the core of prion protein that cause a group of neurodegenerative diseases. [more]Prion - Community and Resources

Noun
1. (microbiology) an infectious protein particle similar to a virus but lacking nucleic acid; thought to be the agent responsible for scrapie and other degenerative diseases of the nervous system
(hypernym) particle
(classification) microbiology

An infectious agent which does not have any nucleic acid (but just protein). Responsible for scrapie in sheep, kuru and Creutzfeldt-Jakob disease in humans.