tailieunhanh - Ebook Medical microbiology: Part 2

(BQ) Part 2 book "Medical microbiology" presentation of content: General properties of viruses, pathogenesis and control of viral diseases, hepatitis viruses, paramyxoviruses and rubella virus, human cancer viruses, aids and lentiviruses, medical mycology, medical parasitology, principles of diagnostic medical microbiology, cases and clinical correlations,.and other contents. | SECTION IV  VIROLOGY 29 C General Properties of Viruses Viruses are the smallest infectious agents (ranging from about 20 to 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as their genome. The nucleic acid is encased in a protein shell, which may be surrounded by a lipid-containing membrane. The entire infectious unit is termed a virion. Viruses are parasites at the genetic level, replicating only in living cells and are inert in the extracellular environment. The viral nucleic acid contains information necessary to cause the infected host cell to synthesize virus-specific macromolecules required for the production of viral progeny. During the replicative cycle, numerous copies of viral nucleic acid and coat proteins are produced. The coat proteins assemble together to form the capsid, which encases and stabilizes the viral nucleic acid against the extracellular environment and facilitates the attachment and penetration by the virus upon contact with new susceptible cells. The virus infection may have little or no effect on the host cell or may result in cell damage or death. The spectrum of viruses is rich in diversity. Viruses vary greatly in structure, genome organization and expression, and strategies of replication and transmission. The host range for a given virus may be broad or extremely limited. Viruses are known to infect unicellular organisms, such as mycoplasmas, bacteria, and algae, and all higher plants and animals. General effects of viral infection on the host are considered in Chapter 30. Much information on virus–host relationships has been obtained from studies on bacteriophages, the viruses that attack bacteria. This subject is discussed in Chapter 7. Properties of individual viruses are discussed in Chapters 31–44. H A P T E R TERMS AND DEFINITIONS IN VIROLOGY Schematic diagrams of viruses with icosahedral and helical symmetry are shown in Figure 29-1. Indicated viral components are described below. Capsid: