Proteins are made of folded chains of amino acids: prions are misfolded proteins, causing a variety of 'prionic' diseases which attack the brain.
In technical language, prion diseases are a family of rare progressive neurodegenerative disorders affecting both humans and animals.
That means the prion diseases begin slowly and get worse over time, attacking the nervous system of the animal they attack.
Known prionic diseases include kuru, Creutzfeldt-Jakob disease (and vCJD) in humans, scrapie in sheep, CWD in deer and BSE or 'mad cow disease' in cattle.
Scrapie was the first disease known, and for a very long time, scientists believed that the disease could not cross what they called 'the species barrier'.
Scientists now believe that BSE was caused when ground-up nerve tissue from diseased sheep was fed to cattle as a food supplement.
Scientists strongly suspect that vCJD (variant Creutzfeldt-Jakob disease) in humans was caused by eating cows suffering from BSE.
Other human prion diseases are: GSS (Gerstmann-Straussler-Scheinker syndrome); FFI (Fatal familial Insomnia) and Alpers Syndrome.
Prions can pass on their misfolding to other proteins, causing prionic diseases to spread. The only answer is to destroy all carcases of diseased animals.