Viruses are obligate intracellular parasites whose replication depends absolutely on their hosts. This interplay has important consequences, both for the virus and the host, and it is important to consider the interaction from the point of view of both virus and host. This book is divided into two parts. The first seven chapters dealt with the viruses themselves—their composition, their genomes, and their replication strategies—and considered their interactions with their host cells only in simple ways (discussing, for example, the synthesis of viral RNA and proteins in the context of the viral life cycle in a single cell). The second part of the book will consider in more detail the complexities of the interactions of retroviruses and their host organisms; this small section is intended to help the reader make the transition between these two different ways of thinking about retroviruses.

Retroviruses are small in the genetic sense, having genomes that are only about 10 kb in length, and a correspondingly small complement of virion proteins (Chapter 2). A virus that carries such a small complement of genetic information depends heavily on the host cell for essential replication functions. To replicate, retroviruses have a number of simple and direct requirements, most of which have already been discussed in the first seven chapters. First, the host cell must express a specific receptor on its surface to provide a site for the virus to bind and trigger the entry process, mediated by the viral Env protein (Chapter 3). Second, the cell must supply deoxynucleotides in adequate concentration for the virion reverse transcriptase to transform the RNA genome into DNA (Chapter 4). Third, there must be a means for the viral DNA to access host chromosomes as targets for viral integration; integration may also require the aid of host repair enzymes (Chapter 5). Fourth, host machinery and components are necessary to express viral RNA and carry out the processing (polyadenylation and splicing) and transport of both viral genomic and messenger RNA to the cytoplasm (Chapter 6). Finally, host cell machinery is necessary for the synthesis, folding, modification, and transport of viral proteins to the membrane assembly sites (Chapter 7).

Because, in its DNA (proviral) form, the retroviral genome mimics a cellular gene, the interaction of the viral genome with the host machinery involved in gene regulation and expression is extensive, and different retroviruses have adapted themselves (or been adapted by virologists) to exploit this complex host machinery in different ways, providing a wealth of biological phenomena for study. First, retroviruses can become integrated into germ line DNA as endogenous viruses, leading to permanent genetic consequences for the descendants of the original host, a property they share with a variety of nonviral, but related, reverse-transcriptase-containing elements (Chapter 8). Second, the special replication properties of retroviruses permit development of vectors for modifying the genetic composition of the host (Chapter 9). Third, retroviruses display a remarkable variety of pathogenic interactions with their host (Chapter 10). Retrovirally induced diseases can be acute, episodic, and chronic, appearing soon after infection or after a long latent period; there are also lifelong infections that have no obvious pathogenic consequences; the most well known of the retroviral diseases are the immunodeficiencies associated with HIV and its close relatives (Chapter 11). Finally, the dependence of the virus on the host presents a major problem in developing antiviral strategies (Chapter 11); since so much of viral replication depends on host factors, there are relatively few viral targets, and the variability of the targets makes them difficult to access effectively.