VIRAL INFECTIONS AND ANTIVIRAL AGENTS
Many human diseases are due to viruses. These agents consist of genomes of either DNA or RNA inside a protein shell. Despite this deceptive simplicity, virus infections are less well understood than bacterial diseases, largely because viruses cannot be grown alone in culture but depend on a host cell. Until recently, protection against virus diseases relied on public health measures and vaccination.
Only since the late 1980s have a significant number of specific antiviral agents become available. Pathogenic bacteria contain many unique components not found in eukaryotic cells, which can be targeted by antibiotics. In contrast, because viruses rely on the host cell for almost all of their metabolic reactions, they usually have few unique components apart from the structural proteins of the virus particle.
Consequently, most chemical agents that prevent virus metabolism are also toxic to the host cells. Another problem is that viruses mutate rapidly and so develop resistance to antiviral agents relatively quickly. This is especially serious for RNA viruses, such as influenza or HIV, which have extremely high mutation rates. Like pathogenic bacteria, viruses must also attach to and invade host cells. Recognition proteins on the surface of the virus capsid bind to specific receptors on the surface of the host cell. After entry, viral replication occurs at the expense of the host cell, which supplies not only raw material and energy, but also the ribosomes needed for synthesis of viral proteins and often many of the enzymes required for synthesis of viral nucleic acids as well.
Finally, new virus particles are assembled and exit the cell. These stages, and some corresponding antiviral agents are shown in Figure 21.1 and listed in Table 21.1. Antiviral agents that combat HIV are discussed in the later section on AIDS. In addition to HIV, we have chosen to focus on influenza, as it is one of the most widespread human viruses and illustrates many facets of virus biology and of the use of biotechnology for virus control. Finally, we discuss prions. These infectious agents were originally believed to be anomalous viruses, hence their inclusion here. However, they consist solely of protein, with no enclosed nucleic acid. Thus, they are definitely not viruses despite sharing the superficial properties of size and infectiousness. Indeed, recent work suggests prion disease is related to other neurological disorders, not normally regarded as infectious.
Relatively few antiviral agents are available compared to the number of antibiotics for treating bacterial infections. Moreover, most antivirals have harmful side effects.