Virus Structure describes the physical characteristics of isolated viruses that represent typical structural groups, with particular reference to those features analyzed with the aid of the electron microscope. For descriptive purposes, the book has been divided into sections starting with the small icosahedral viruses and leading to the larger and more sophisticated structures, regardless of whether they are animal, plant, or bacterial viruses. These include double-stranded DNA icosahedral viruses, herpesvirus, viruses with helical symmetry, and viruses with complex or a combination of symmetries. Many common architectural features will be found in those viruses selected for discussion in each of the sections, and for these reasons the introduction places some emphasis on the symmetry elements rather than the shapes of viruses. The mechanism by which viruses enter host cells and the events that follow once the cell has been infected are only mentioned briefly as the virus-host interaction is a relatively complex one.