Influenza natural history, complications and prognosis

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For more information about non-human (variant) influenza viruses that may be transmitted to humans, see Zoonotic influenza

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: ; Ammu Susheela, M.D. [2]


The incubation period of influenza is 1 to 4 days. Uncomplicated influenza can present with constitutional symptoms and can resolve within 3-7 days; while more complex disease may be observed with patients developing pneumonia, otitis, encephalitis and sepsis.

Natural History


  • Influenza viruses are spread from person to person primarily through large-particle respiratory droplet transmission
  • Contact with respiratory-droplet contaminated surfaces is another possible source of transmission.
  • Airborne transmission (via small-particle residue [less than or equal to 5µm] of evaporated droplets that might remain suspended in the air for long periods of time) also is thought to be possible, although data supporting airborne transmission are limited.

Incubation Period

  • The typical incubation period for influenza is 1-4 days (average: 2 days).
  • Adults shed influenza virus from the day before symptoms begin through 5-10 days after illness onset.
  • Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms. Severely immunocompromised persons can shed virus for weeks or months.


  • Uncomplicated influenza illness typically resolves after 3-7 days for the majority of persons, although cough and malaise can persist for >2 weeks.
  • Influenza virus infections can cause primary influenza viral pneumonia; exacerbate underlying medical conditions (e.g., pulmonary or cardiac disease); lead to secondary bacterial pneumonia, sinusitis, or otitis media; or contribute to coinfections with other viral or bacterial pathogens.
  • Young children with influenza virus infection might have initial symptoms mimicking bacterial sepsis with high fever, and febrile seizures have been reported in 6%-20% of children hospitalized with influenza virus infection.
  • Respiratory illnesses caused by influenza virus infection are difficult to distinguish from illnesses caused by other respiratory pathogens on the basis of signs and symptoms alone.
  • Young children are less likely to report typical influenza symptoms (e.g., fever and cough).
  • In the United States, annual epidemics of influenza typically occur during the fall or winter months, but the peak of influenza activity can occur as late as April or May
  • Influenza-related complications requiring urgent medical care, including hospitalizations or deaths, can result from the direct effects of influenza virus infection, from complications associated with age or pregnancy, or from complications of underlying cardiopulmonary conditions or other chronic diseases.
  • Influenza viruses cause disease among persons in all age groups.
  • Rates of infection are highest among children, but the risks for complications, hospitalizations, and deaths from influenza are higher among persons aged 65 years and older, young children, and persons of any age who have medical conditions that place them at increased risk for complications from influenza.


Possible complications of influenza, especially for those at high risk, include:


Anyone at any age can have serious complications from the flu, but those at highest risk include:

In most individuals who are otherwise healthy, the flu goes away within 7 to 10 days.[1]


  1. "Centers for Disease Control and Prevention (CDC) Influenza".

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