Inherited resistance to animal viruses may be conveniently classified into three types: monogenetic, following simple mendelian ratios; polygenetic; and cytoplasmic. A virus is a unique cellular parasite, dependent upon the host for reproduction and nourishment in a variety of different ways. Since, as with the other types of parasites, the host and the parasite have necessarily evolved together. It is a distortion to consider the resistance of the host, without considering the evolutionary steps in the development of this extreme form of parasitism; therefore, this chapter reviews some of the ideas put forward about host-agent interactions in plants as well as in animals. The importance of genes in regulating the resistance to disease, including parasites and parasitoids, is apparent if the disease is considered to be an important evolutionary force. The selective effects of viruses have not yet been adequately studied. Continued attempts to find a correlation between the different blood groups and differing severity of smallpox infection clearly searched for selective forces, but the results were inconclusive. Most of the knowledge of genetic resistance to virus disease rests on the study of resistance to selected agents in various inbred strains of mice and chickens, rather than on any knowledge of the effects of genetic resistance in a natural heterozygous population. The increasing frequency, however, with which genetic resistance is found, is in itself an evidence that these genes are important in natural outbred populations. In addition, there are increasing numbers of virus diseases, in which the viral agent seems to be inherited in a mendelian fashion.