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(BQ) Part 2 book "Harrison's infectious disease" presents the following contents: Viral infections, prion diseases, fungal and algal infections, protozoal and helminthic infections, laboratory values of clinical importance. | VIRAL INFECTIONS SECTION V PART 1 VIRAL DISEASES GENERAL CONSIDERATIONS CHAPTER 73 MEDICAL VIROLOGY Fred Wang Elliott Kieff 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 generation of high-energy phosphates. Typically viral nucleic acids encode proteins necessary for replicating and packaging their nucleic acids within the biochemical milieu of host cells. Viruses differ from viroids prions and virusoids. Viru-soids are nucleic acids that depend on helper viruses to package their nucleic acids into virus-like particles. Viroids are naked cyclical mostly double-stranded small RNAs. Viroids appear to be restricted to plants spread from cell to cell and are replicated by cellular RNA polymerase II. Prions Chap. 101 are abnormal protein molecules that can spread. These molecules reproduce by changing the structure of their normal cellular protein counterparts. Prions have been implicated in neurode-generative conditions such as Creutzfeldt-Jakob disease Gerstmann-Straussler disease kuru and human bovine spongiform encephalopathy mad cow disease . VIRAL STRUCTURE Viruses have from a few to several hundred genes.These genes may be in a single-strand or double-strand DNA genome or in a single-strand sense a single-strand or segmented antisense or a double-strand segmented RNA genome. Sense-strand RNA genomes can be translated directly into protein. Sense and antisense genomes are also referred to as positive-strand and negative-strand genomes respectively. The viral nucleic acid is usually associated with one or more virus-encoded nucleoproteins in the core of the viral particle. The viral nucleic acid and nucleoproteins are almost always enclosed in a protein shell called a .
