Prion protein can become infectious and cause neurodegenerative disease. Here four nerve cells in a mouse illustrate how infectious prion protein moves within cells along neurites - any projection from the cell body of a neuron. This projection can be either an axon or a dendrite. A prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents (virus/bacteria/fungus/parasite) which must contain nucleic acids (either DNA, RNA, or both). The word prion is derived from the words protein and infection. Prions are responsible for the transmissible spongiform encephalopathies in a variety of mammals, including bovine spongiform encephalopathy (BSE, mad cow disease) in cattle and Creutzfeldt-Jakob disease (CJD) in humans. All known prion diseases affect the structure of the brain or other neural tissue and all are currently untreatable and universally fatal. All known mammalian prion diseases are caused by the prion protein, PrP.