Prions

Prions are composed of only modified host proteins cause conditions such as spongiform encephalopathies including kuru (associated with human cannibalism) and Creutzfeldt-Jakob disease (CJD) which is associated with corneal transplants. Severe cases of CJD occurred in England, where more than 100,000 cattle have died of prion-associated bovine spongiform encephalopathy, better known as "mad cow" disease. Infectious prion proteins (PrP^sc) are not viruses, because they lack RNA and DNA. PrP^sc proteins are protease resistant can combine with normal protease-sensitive host proteins PrP^c that are present on the surface of neurons. Infectious PrP^sc proteins that induce a conformational change on the normal PrP^c proteins, and abnormal complexes are internalized into neuorns, damaging cells and causing the spongiform encephalopathy.