Prion diseases, also known as spongiform encephalopathies, are a group of transmissible disorders that affect the brains of humans and animals and are invariably fatal. The nature of the pathogenic agent is unique. In contrast to all other known infectious agents (for example, viruses, bacteria, yeast), which contain nucleic acids as information carriers, the infectious prion pathogen seems to lack this component. The prevailing view is that the molecule responsible for the transmission of the spongiform encephalopathies is the structural isoform of a protein, named prion protein (the term prion is a skewed acronym for proteinaceous infectious agent). The identification of an infectious agent that apparently does not require a nucleic acid for replication presents a fascinating puzzle in modern biology and medicine.