The study of animal virus pathogenesis - how viruses cause disease - is an important and rapidly developing area, especially so since the development of AIDS and the consequent increase in research activity. Viruses can cause disease in two distinct ways. They can alter the cell and damage it directly by shutting down its basic mechanisms or by producing toxins or by breaking the cell membrane. Alternatively the virus can alter a cell’s function without destroying it, for instance, in the case of a nerve cell the virus may alter synthesis of neurotransmitters. In contrast to the stricken cells damaged by viral infection, in this second type of infection the cells appear normal when examined by physical means, e.g. microscopy, and by most biochemical assays, e.g. total cellular RNA. Understanding viral pathogenesis requires knowledge and use of techniques drawn from several disciplines from researchers working at the interface between virology and immunology. This volume charts the way that viral tropism, viral expression , and viral infectivity are assayed. It describes the identification of antiviral cytotoxic T cells, natural killer and free antibody responses. The final chapters deal with the latest methods for raising antipeptide antibodies and making vaccinia/viral cDNA for immunologic target recognition, tissue and whole animal in situ hybridization.