Pathogenesis of Viral Infections and Diseases

Viruses first infect their host, then spread, and then damage their target tissues. To ensure their perpetuation, viruses must be transmitted to other susceptible individuals—that is, they must be shed with secretions or excretions into the environment, be taken up by another host or a vector, or be passed congenitally from mother to offspring. Viruses have developed a remarkable variety of strategies to ensure their own survival. Individual viruses cause their associated diseases through a considerable variety of distinct pathogenic mechanisms. Viruses differ greatly in their virulence, but even in a population infected by a particular virus strain, there are usually striking differences in the outcome of infection of individual animals. Similarly, there is much variation among viruses of the same species, and the determinants of viral virulence are often multigenic, meaning that several viral genes contribute to the virulence of individual viruses. The determinants of host resistance/susceptibility are usually multifactorial and include not only a variety of host factors but environmental ones as well. There is wide variation in the virulence of viruses, ranging from those that almost always cause inapparent infections, to those that usually cause disease, to those that usually cause death. Meaningful comparison of the virulence of viruses requires that factors such as the infecting dose of the virus and the age, sex, and condition of the host animals and their immune status be equal; however, these conditions are never met in nature, where heterogeneous, outbred animal populations are the rule and the dynamics of exposure and viral infection are incredibly varied.