ICD-10-CM Code A81
Atypical virus infections of central nervous system

A prion (i/ˈpriːɒn/) is an infectious agent thought to be the cause of the transmissible spongiform encephalopathies (TSEs). It is 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. The word prion, coined in 1982 by Stanley B. Prusiner—derived from the words protein and infection, hence prion—is short for "proteinaceous infectious particle", in reference to its ability to self-propagate and transmit its conformation to other proteins. 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 recent 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.