21.1 Viral Evolution, Morphology, and Classification
Viruses are tiny, noncellular entities that usually can be seen only with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate either by using the replication proteins of a host cell or by using proteins encoded in the viral genome. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA.
21.2 Virus Infections and Hosts
Plant viruses may be transmitted either vertically from parent reproductive cells or horizontally through damaged plant tissues. Viruses of plants are responsible for significant economic damage in both crop plants and plants used for ornamentation. Animal viruses enter their hosts through several types of virus-host cell interactions and cause a variety of infections. Viral infections can be either acute, with a brief period of infection terminated by host immune responses, or chronic, in which the infection persists. Persistent infections may cause chronic symptoms (hepatitis C), intermittent symptoms (latent viruses such a herpes simplex virus 1), or even be effectively asymptomatic (human herpesviruses 6 and 7). Oncogenic viruses in animals have the ability to cause cancer by interfering with the regulation of the host cell cycle.
21.3 Prevention and Treatment of Viral Infections
Viruses cause a variety of diseases in humans. Many of these diseases can be prevented by the use of viral vaccines, which stimulate protective immunity against the virus without causing major disease. Viral vaccines may also be used in active viral infections, boosting the ability of the immune system to control or destroy the virus. A series of antiviral drugs that target enzymes and other protein products of viral genes have been developed and used with mixed success. Combinations of anti-HIV drugs have been used to effectively control the virus, extending the lifespans of infected individuals. Viruses have many uses in medicines, such as in the treatment of genetic disorders, cancer, and bacterial infections.
21.4 Other Acellular Entities: Prions and Viroids
Prions are infectious agents that consist of protein, but no DNA or RNA, and seem to produce their deadly effects by duplicating their shapes and accumulating in tissues. They are thought to contribute to several progressive brain disorders, including mad cow disease and Creutzfeldt-Jakob disease. Viroids are single-stranded RNA pathogens that infect plants. Their presence can have a severe impact on the agriculture industry.