Prion
Prions cause many forms of encephalitis, or brain disease, such as scrapie, Creutzfeldt-Jakob Disease, kuru, and Bovine Spongiform Encephalopathy, also known as Mad Cow Disease. Prions work by changing the shape of proteins in the living things it causes disease in. While normal proteins have lots of alpha helices, or twisted parts, changed proteins have lots of beta sheets, or flat parts. The word is pronounced 'pree-on'.