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High-Yield Facts


  • Influenza usually causes yearly winter epidemics in temperate climates, but new strains that have the potential to cause infrequent but severe worldwide pandemics can arise.

  • Influenza is transmitted by inhalation of infected droplets and aerosols (from persons with coughing or sneezing) or by direct contact with contaminated animals or objects.

  • Viral shedding of influenza begins 24 hours before the onset of clinical illness and can last for 1 to 2 weeks.

  • The most common symptoms in teenagers and adults include fever, cough, headache, sore throat, and malaise. Children may present with atypical symptoms that include lower respiratory tract symptoms. Infants may present with fever or apnea.

  • In patients with lower respiratory symptoms, it may be difficult to distinguish primary influenza pneumonia from secondary bacterial pneumonia (both clinically and radiographically).

  • Most influenza infections are self-limited and require only supportive care; however, antiviral medications can be considered for children at risk for severe or complicated infections (e.g., immunocompromised or with underlying cardiopulmonary disease), healthy children with severe symptoms, or children with special environmental circumstances (e.g., immunocompromised family members).

  • If antiviral medications are started, they should be started within the first 24 to 48 hours of symptoms and given for 5 days. Evidence suggests they shorten the duration of symptoms but it is less clear whether they prevent serious complications, such as viral or secondary bacterial pneumonia.

  • The website of the Centers for Disease Control (CDC),, is regularly updated, and provides information for parents and patients, and recommendations for physicians (e.g., immunization and antiviral medication information).


The name influenza originated in the fifteenth century from the Italian word influenza, meaning influence. It was thought that the disease was due to adverse astrological influences. Symptomatic influenza infection is commonly called “flu.” Flu can affect individuals of any age.


The virus is a member of the Orthomyxoviridae family, typically spheroid or ovoid in shape, and approximately 80 to 120 nm in diameter (Fig. 63-11 and 63-22). It has multiple segments of single-stranded, negative-sense RNA. Influenza A and B have eight gene segments, whereas influenza C has seven gene segments. The genome is surrounded by an envelope, made of a lipid bilayer, within which are embedded matrix proteins M1 (in both influenza A and B) and M2 (in only influenza A). The M1 proteins are located on the interior surface of the envelope and provide stability to the virus, whereas the M2 proteins serve as ion channels that facilitate uncoating of the virus in the host lysosome. The envelope is studded with glycoproteins, approximately 80% of which are hemagglutinin (HA) and approximately 20% are neuraminidase (NA). HA is the viral molecule that binds to the host sialic acid sugar, enabling attachment of the virus to the host epithelial cell. Once attachment has taken place, the process of endocytosis allows for entry of the virus into the cell. HA is the major stimulus for the ...

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