Transmissible spongiform encephalopathies are diseases caused by prions. Prions differ from viruses in that they are small, infectious pathogens that do not contain nucleic acid. Recognized spongiform encephalopathies include Creutzfeldt-Jakob Disease (CJD), variant Creutzfeldt-Jakob Disease (vCJD), Kuru, fatal familial insomnia (FFI), and Gerstmann-Straussler syndrome (GSS). Common characteristics of these diseases include dementia, ataxia, and myoclonus. Unfortunately, these diseases are associated with long incubation periods (20+ years) and once symptoms occur, rapidly progress to death.