Abstract
Although some viruses can infect and cause disease in many species, many are host specific. Within a susceptible host species, there is often a striking difference between individual animals in their levels of resistance. Within susceptible species, resistance varies not only with the genetic constitution of the host but also with age, nutritional status, stress, and many other factors. These genetic and physiological factors determine what is called the nonspecific or innate resistance of the host, in contrast to the immunologically specific resistance that results from the operation of the immune response. Genetic differences in susceptibility are most obvious when different animal species are compared. Common viral infections often tend to be less pathogenic in their natural host species than in certain exotic or introduced species. Immunological responsiveness to particular antigens differs greatly from one strain of mouse to another, being under the control of specific immune response (Ir) genes. There are many of these genes, most of them situated in the region known as the major histocompatibility complex (MHC).
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