from the CDC
http://www.cdc.gov/ncidod/dvrd/prions/
About Prion Diseases
Prion diseases or transmissible spongiform encephalopathies (TSEs) are a family of rare progressive neurodegenerative disorders that affect both humans and animals. They are distinguished by long incubation periods, characteristic spongiform changes associated with neuronal loss, and a failure to induce inflammatory response.
The causative agent of TSEs is believed to be a prion. A prion is an abnormal, transmissible agent that is able to induce abnormal folding of normal cellular prion proteins in the brain, leading to brain damage and the characteristics signs and symptoms of the disease. Prion diseases are usually rapidly progressive and always fatal.
Related conditions: prion diseases
NIH
http://www.ghr.nlm.nih.gov/condition/prion-disease