In discussing the epidemiology and pathogenesis of viral infections it is often convenient to use groupings of viruses based on the routes of transmission. These are not taxonomic groupings, but it is important to understand the context in which the terms are traditionally employed.
Enteric viruses, which replicate primarily in the intestinal tract, are acquired by ingestion of material contaminated with feces. These viruses usually remain localized in the intestinal tract, rather than becoming generalized. Enteric viruses that cause gastroenteritis in humans include the rotaviruses, caliciviruses, astroviruses, and some adenoviruses and coronaviruses. Many enteroviruses that are acquired by ingestion and replicate first in the gut, such as (he polioviruses, do not cause gastroenteritis but may cause generalized diseases.
Respirator}/ viruses are usually acquired by inhalation of droplets and replicate in the respiratory tract. The term is usually restricted to those viruses that remain localized in the respiratory tract, including the orthomyxoviruses, rhinoviruses, and some of the paramyxoviruses, coronaviruses, and adenoviruses Viruses (hat infect via the respiratory tract but cause generalized infections, such as measles and mumps viruses, are not normally called respiratory viruses.
Arboi'irusa (arthropod-borne viruses) infect arthropods that ingest vertebrate blood; they replicate in the tissues of the arthropod and can then be transmitted by bite to susceptible vertebrates. Viruses that belong to five families are included: all orbiviruses (a genus of the reoviruses), most bun-yaviruses, flaviviruses, and togaviruses, and some rhabdoviruses.
Sexually transmitted viruses include some herpesviruses and papillomaviruses thai cause lesions in flic genital tract, as well as certain retroviruses and hepatitis viruses that are often transmitted during sexual activity but cause generalized disease.
Hepafitis viruses are olten considered together bccause the liver constitutes the principal target. Formerly including the viruses of yellow fever and Rift Valley fever, the term is now generally restricted to hepatitis A, R, C, D, and E viruses. Epidemiological^ they are diverse, for hepatitis A and E are spread by the enteric route, whereas hepatitis B, C, and D are transmitted parcn-terally (by blood) or sexually Because each hepatitis virus belongs to a tax-onomically different family, each is considered in a different chapter in Part II of this book
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