Classification of Viruses

Viruses are composed of one or more strands of a nucleic acid (core) enclosed by a protein coat (capsid). Many viruses possess an outer envelope of protein or lipoprotein. Viral cores can contain either DNA or RNA; thus, viruses may be classified as DNA viruses or RNA viruses. Further classification is usually based on morphology, cellular site of viral multiplication, or other characteristics.

Examples of DNA viruses and the diseases that they produce include adenoviruses (colds, conjunctivitis); hepadnaviruses (hepatitis B); herpesviruses (cytomega-lovirus, chickenpox, shingles); papillomaviruses (warts); and poxviruses (smallpox). Pathogenic RNA viruses include arborviruses (tick-borne encephalitis, yellow fever); arenaviruses (Lassa fever, meningitis); or-thomyxoviruses (influenza); paramyxoviruses (measles, mumps); picornaviruses (polio, meningitis, colds); rhab-doviruses (rabies); rubella virus (German measles); and retroviruses (AIDS).

virus. An overview of the viral replication cycle is shown in Figure 50.1.

Infection begins when specific receptor sites on the virus recognize corresponding surface proteins on the host cell. The virus penetrates the host membrane by a mechanism resembling endocytosis and is encapsulated

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