An historical introduction to animal virology. A general outline of the problems of animal virology. The normal cellular and tissue background to virus infection. The physical and chemical structure of infective virus particles. The initiation of cellular infection. Multiplication and liberation of virus. Morphological and functional changes in cells infected with viruses. Interference between viruses and other aspects of intracellular replication. The pathogenesis of virus disease. The significance of immunity in virus disease. The detection, titration, and properties of neutralizing antibodies. Active and passive immunization against virus diseases. Latent infections. Viruses in relation to neoplastic disease. Ecological aspects of virus disease. Influence of ecological circumstances on the character of the virus. Variation in animal viruses. Recombination and other genetic interactions in animal viruses. The evolution of animal viruses. The classification and nomenclature of animal viruses.