Viruses are the smallest microorganisms causing human diease. Their genome is composed of either DNA or RNA; the complete base sequence of nearly all human virus genomes is now known and genome structure is used as basis of virus classification. The virus genome is enclosed in a protein shell, and in some cases a lipid envelope. Unlike bacteria which can grow in cell-free medium, all viruses are obligate intracellular pathogens. Glycoproteins on the virus
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surface recognise specific cellular receptors, whose distribution is often a major determinant of which cells the virus infects (its tropism), and hence of the type of disease produced. Table 2.10 presents the current classification of the viruses which cause human disease. This section covers the major viral diseases not dealt with elsewhere in the book; they a presented here by virus family, and are complemen the discussion of infective syndromes due to other sections-see particularly those on meningitis and encephalitis , respiratory infections, hepatitis and enteric infections .