This symposium book covers the central factors that influence the pathogenicity of virus and prion infections. Virus and prion diseases remain a major public health threat, in both developed and developing countries. The worldwide HIV pandemic is one example of a newly emerged virus disease, and older human viruses such as influenza, papilloma, and the hepatitis viruses still cause major health problems. Some viruses may also establish persistent infections which can lead to the development of chronic diseases, including cancer.