This book was initiated and edited by general microbiologists or virologists, who had numerous questions concerning prion origin and replication, virulence and pathogenicity, diagnosis and therapy, transmission and susceptibility, as well as inactivation and prevention. Although the rapid progress of research had outdated the ‘state of the art’ at the time of publication, those questions were of such common interest that the book was soon sold out. Now, three years later, a second approach to these issues is presented. The protein-only hypothesis on prions is now commonly accepted, although there are some reservations. Therefore, the contribution dealing with arguments against this hypothesis was not rewritten. While the BSE epidemic has declined, the number of new, i.e. variant CreutzfeldtJacob disease cases has risen. Although it will probably not reach the features of an epidemic in humans, the challenge for science, medicine and public health is always present. After the first American case of BSE emerged in a cow that had not been imported, the question about an endogenous or exogenous origin of disease has again been raised. Our knowledge of prion biology has considerably improved covering both human, animal and even microbe affections. Beside some insights in basic research, the second edition emphasizes practical aspects of fighting human and animal prion diseases. Additionally, chapters on public regulations of this issue and on veterinary measures were included. The volume editors wish to thank Karger Publishers and the series editor of Contributions to Microbiology for encouraging this second edition.