Transmission electron micrograph (TEM) of negative stained fibrils in an extract of fresh brain from a cow infected with bovine spongiform encephalopathy, a form of transmissible, slow virus encephalopathy. Human slow virus encephalopathies include Kuru, Creutzfeldt-Jakob disease and Gerstmann-Straussler syndrome. The agents causing these diseases form a group of virus-like organisms called prions. The fibrils are believed to be aggregations of the major proteins of the infectious agents themselves. Magnification: x205,000 at 6x8-inch size, x36,300 at 35mm size.