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Ebook Harrison’s infectious diseases (2/E): Part 2

Part 2 book “Harrison’s infectious diseases” has contents: Herpes simplex virus infections, molluscum contagiosum, monkeypox, and other poxvirus infections, common viral respiratory infections, human papillomavirus infections, parvovirus infections, viral gastroenteritis, acute viral hepatitis, chronic hepatitis, and other contents. | SECTION V VIRAL INFECTIONS cHaPter 82 MEDICAL VIROLOGY Fred Wang ■ Elliott Kieff a capsid. Because of the limited genetic complexity of viruses, their capsids are usually composed of multimers of identical capsomeres. Capsomeres are in turn composed of one or a few proteins. Capsids have icosahedral or helical symmetry. Icosahedral structures approximate spheres but have two-, three-, and fivefold axes of symmetry, while helical structures have only a twofold axis of symmetry. The entire structural unit of nucleic acid, nucleoprotein(s), and capsid is called a nucleocapsid. Many human viruses are composed simply of a core and a capsid. For these viruses, the outer surface of the capsid mediates contact with uninfected cells. Other viruses are more complex and have an outer lipid-containing envelope derived from virus-modified membranes of the infected cell. The piece of infected-cell membrane that becomes the viral envelope has usually been modified during infection by the insertion of virus-encoded glycoproteins, which mediate contact of enveloped viruses with uninfected cells. Matrix or tegument proteins fill the space between the nucleocapsid and the envelope in many enveloped viruses. In general, enveloped viruses are sensitive to lipid solvents and nonionic detergents that can dissolve the envelope, while viruses that consist only of nucleocapsids are somewhat resistant. A schematic diagram for large and complex herpesviruses is shown in Fig. 82-1. Prototypical pathogenic human viruses are listed in Table 82-1. The relative sizes and structures of typical pathogenic human viruses are shown in Fig. 82-2. DefininG a VirUS Viruses consist of a nucleic acid surrounded by one or more proteins. Some viruses also have an outer-membrane envelope. Viruses are obligate intracellular parasites: they can replicate only within cells since their nucleic acids do not encode the many enzymes necessary for protein, carbohydrate, or lipid metabolism and for the .