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  • Influenza is an enveloped single-stranded RNA virus found in animals and humans, which is one of the most important causes of respiratory illness throughout the world.
  • 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.
  • Although there are only three main serotypes, new strains of influenza are regularly formed by mutations and genetic reassortment, which change the viral surface antigens, thereby facilitating spread in populations that do not have antibody.
  • 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.
  • The incubation period is 1 to 4 days (average of 2 days).
  • 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 and “rule out sepsis” or with apnea.
  • In patients with lower respiratory symptoms, it may be difficult to distinguish primary influenza pneumonia from secondary bacterial pneumonia (both clinically and radiographically).
  • Routine laboratories, such as CBC, are usually less helpful than culture or PCR of nasopharyngeal secretions for influenza and other respiratory viruses.
  • Most influenza infections are self-limited and require only supportive care; however, antiviral medications can be considered for the following:
    • Children who are at risk for severe or complicated infection (such as immunocompromised children and children with underlying cardiopulmonary disease)
    • Healthy children with severe symptoms
    • Children with special environmental circumstances (such as 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. They shorten the duration of symptoms, but prevention of serious complications, such as viral or secondary bacterial pneumonia, is less well documented.
  • In the clinic or emergency department, droplet precautions as well as standard precautions should be used, with close attention paid to good hand-washing.
  • The Web site of the Centers for Disease Control (CDC),, is regularly updated, and provides the following:
    • Helpful information sheets that are easily downloaded for parents and patients
    • Current recommendations for physicians, including immunization and antiviral medication information
  • Information regarding local epidemiology and outbreaks can be obtained from local health departments.


Symptomatic influenza infection is commonly called “flu.” Flu can affect individuals of any age. The name influenza originated in the 15th century from the Italian word influenza, meaning influence. It was thought that the disease was due to adverse astrological influences. It was first used in English in the 1700s.


The virus is a member of the Orthomyxoviridae family, typically spheroid or ovoid in shape, and approximately 80 to 120 nm in diameter (Figs. 60–1 and 60–2). It has multiple segments of single-stranded, negative-sense ...

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