Any transmissible neurodegenerative disease believed to be caused by a proteinaceous infectious particle (also known as prion proteins, or PrPs). PrPs change other cellular proteins, producing intracellular vacuoles (“spongiform change”) that disrupt the functioning of neurons. Included in this group are Creutzfeldt-Jacob disease, Gerstmann-Strussler-Scheinker syndrome, kuru, and fatal familial insomnia in humans, mad cow disease (bovine spongiform encephalopathy), and scrapie in sheep and goats. Prion diseases may be transmitted by hereditary changes in the gene coding PrP; by contaminated biological agents such as plasma or serum, human growth hormone, and organ transplants; and possibly, by eating the flesh of infected animals. All prion diseases are characterized by a long incubation period, followed by a rapidly progressive dementia.