In 1982, Stanley Prusiner publishes in Science a paper challenging one of biology's most firmly established dogmas: that every infectious agent needs nucleic acid — DNA or RNA — to replicate. After a decade researching the agent causing scrapie, a neurodegenerative disease of sheep, Prusiner succeeds in isolating from infected hamster brains a preparation containing a single infectious agent, and shows through treatments that degrade nucleic acids that this agent retains its full infectivity: it contains neither DNA nor RNA. The agent is a protein, and only a protein, capable of propagating its misfolded pathological form to other identical healthy proteins, in a self-perpetuating cascade. Prusiner coins the term "prion", an acronym for "proteinaceous infectious particle", to name this entirely new biological principle. The hypothesis was received with strong scientific skepticism for more than a decade, since it contradicted the fundamental assumption that replicable biological information always resides in nucleic acids. The consensus changed radically with the "mad cow disease" crisis (bovine spongiform encephalopathy) in the United Kingdom in the 1990s and its transmission to humans as variant Creutzfeldt-Jakob disease, which dramatically confirmed the mechanism Prusiner had proposed. Prions today explain an entire group of fatal neurodegenerative diseases — including scrapie, mad cow disease, kuru, and the human variant of Creutzfeldt-Jakob disease — and also opened a new conceptual framework, that of pathological protein misfolding, relevant to understanding much more common diseases such as Alzheimer's and Parkinson's.