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Influenza viruses are important human pathogens because they cause both outbreaks of influenza that sicken and kill thousands of people each year as well as infrequent but devastating worldwide epidemics (pandemics).

Influenza viruses are the only members of the orthomyxovirus family. The orthomyxoviruses differ from the paramyxoviruses primarily in that the former have a segmented RNA genome (usually eight pieces), whereas the RNA genome of the latter consists of a single piece.1 The term myxo refers to the observation that these viruses interact with mucins (glycoproteins on the surface of cells).

In addition, the orthomyxoviruses are smaller (110 nm in diameter) than the paramyxoviruses (150 nm in diameter). See Table 39–1 for additional differences.

TABLE 39–1Properties of Orthomyxoviruses and Paramyxoviruses

Table 39–2 shows a comparison of influenza A virus with several other viruses that infect the respiratory tract. Table 39–3 describes some of the important clinical features of influenza virus and compares them with the clinical features of the other medically important viruses in this chapter.

TABLE 39–2Features of Viruses That Infect the Respiratory Tract1

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