"The name 'prion' was coined to describe an unknown infectious agent that causes a variety of neurological illnesses in mammals. The term comes from the phrase "proteinaceous infectious particle," which alludes to the once-controversial idea that the infectious agent causing those diseases is made entirely of protein and lacks a nucleic acid genome. A prion is a protein that causes normal proteins in the brain to fold improperly. Prion illnesses can affect both humans and animals, and diseased meat products can sometimes spread the sickness to humans. Creutzfeldt-Jakob disease is the most frequent kind of prion disease that affects humans.