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The information presented in this chapter can be used for navigating this book and for planning a virology course. Figures 3.1 to 3.7 illustrate seven strategies based on viral mRNA synthesis and genome replication and serve as the points of departure for detailed analyses of the principles of virology. For those who prefer to teach virology based on specific viruses or groups of viruses, the material in this chapter can be used to structure individual reading or to design a virology course while adhering to the overall organization of this textbook by function. Reference to this chapter provides answers to questions about specific virus families. For example, Fig. 3.5 provides information about (+) strand RNA viruses and Fig. 3.10 indicates specific chapters in which these viruses are discussed.

Since the earliest days of experimental virology, genetic analysis has been essential for studying viral genomes. Initially, methods were developed to produce viral mutants by chemical or UV mutagenesis, followed by screening for readily identifiable phenotypes. Because it was not possible to identify the genetic changes in such mutants, it was difficult to associate proteins with virus-specific processes. This limitation was surmounted with the development of cloned infectious DNA copies of viral genomes, an achievement that enabled the introduction of defined mutations. These methods for reducing or ablating the expression of specific viral or cellular genes comprise a complete genetic toolbox that provides countless possibilities for studying the viral genome and the interaction of viral gene products with those of the cell. The ability to manipulate cloned DNA copies of viral genomes has also enabled the development of viruses as vectors for the expression of foreign genes, for gene therapy, viral oncotherapy, and to deliver vaccines to prevent infectious diseases. How ironic it is that our study of the viruses that cause disease has led to their transformation into therapeutic agents!