Prion

Prion diseases (TSEs)
Microscopic "holes" are characteristic in prion-affected tissue sections, causing the tissue to develop a "spongy" architecture.
Classification and external resources
Specialty	infectious disease
ICD-10	A81
ICD-9-CM	046
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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 untreatable and universally fatal.

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