Among unconventional agents and unclassified viruses the contributions to this volume focused on prion-related diseases, with special emphasis on bovine spongiform encephalopathy and human spongiform encephalopathies, and Borna disease virus, an agent known since long time to be pathogenic for horses and sheep, which is now discussed as a potential pathogen for humans. Additionally, the volume contains articles about newly discovered viruses like porcine respiratory and reproductive syndrome virus and viruses that are classified only provisionally like African swine fever virus, hepatitis C and E viruses, or the arteriviruses.