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Influenza: Overview and Risk Factors

The influenza virus causes a respiratory infection with symptoms similar to the common cold, but also often causes more severe symptoms, such as high fever, muscle aches, and weakness.

Symptoms may begin abruptly within one to four days of acquiring the infection. However, an infected person can pass the virus to others before symptoms begin and for approximately one week after the start of symptoms. In most cases, the disease resolves on its own. In high–risk populations, however, influenza can be life–threatening.

Influenza has become a matter of increasing concern due to the recent outbreaks of H5N1 avian influenza (“bird flu”). Birds can carry influenza viruses in their digestive tracts and may transmit the viruses to humans.

Risk Factors

  • Contact with infected individuals: Direct contact with persons who have an upper respiratory infection permits transmission of the virus. Coughing or sneezing can spread the virus through the air.
  • Closed settings: Homes and schools have higher transmission rates, compared with typical work settings.
  • Immunocompromise: Persons with compromised immune systems, including those with malnutrition, diabetes, and chronic lung disease, generally have a more severe disease if they are infected by influenza.
  • Winter season: Influenza infections more commonly occur in the winter, but cold climates are not necessarily a risk factor for the disease.
  • Contact with infected birds: Risk for H5N1 influenza is principally related to contact with infected birds, bird feces, or bird products.


Influenza: Diagnosis and Treatment >>