The infectious protein that causes diseases such as Creutzfeldt–Jakob disease is called a prion. Unlike a virus, bacterium, or fungus, a prion contains no DNA or RNA; it is a misfolded form of a protein that can induce normally folded copies to misfold.
In humans, prions are associated with fatal neurodegenerative diseases called transmissible spongiform encephalopathies. In Creutzfeldt–Jakob disease, abnormal prion protein accumulates in nervous tissue, disrupting neurons and eventually causing rapidly progressive problems such as memory loss, impaired coordination, involuntary movements, and dementia.
The word prion was coined in 1982 by neurologist and biochemist Stanley B. Prusiner from “proteinaceous infectious particle.” Prusiner’s work challenged the conventional assumption that every infectious agent must carry genetic material. He received the 1997 Nobel Prize in Physiology or Medicine for discovering prions.
Prion diseases can arise sporadically, through inherited mutations in the PRNP gene, or through exposure to contaminated tissue. Related diseases include scrapie in sheep, chronic wasting disease in deer, and bovine spongiform encephalopathy, commonly called mad cow disease. Prions are not the same as ordinary protein clumps associated with many other diseases, because they can propagate their abnormal shape and, in some circumstances, transmit infection.