Prion disease also known as transmissible spongiform encephalopathies (TSE) are an extremely rare group of diseases affecting humans and animals that all result in brain death with no known cure. They are caused by " an infectious agent known as a prion, which is derived from a misfolded version of a normal host protein known as prion protein." This so far is an almost word for word description of the diseases from the National Institute of Allergy and Infectious Disease, which I did not know existed until writing this.