Synonyms: Flu, Seasonal flu

Medical Specialties: Family practice, Geriatrics, Internal medicine

Clinical Definition

Influenza is a highly contagious respiratory illness caused by viruses that produce mild to severe symptoms that are sometimes life-threatening. While influenza is most prevalent during winter months, ''flu season'' actually starts in late fall and persists until spring. 

In Our Own Words

Influenza viruses are contagious respiratory illnesses that infect your nose, throat and lungs, which explains the most common symptoms: runny nose, persistent cough and sore throat. The virus spreads via droplets produced by infected people when they cough, sneeze or talk. People with the flu can spread it to others up to about six feet away. You may also become infected by touching an object, such as a doorknob or a steering wheel, which contains the flu virus and then become sick after touching your mouth or eyes.


Incidentally, influenza is not the same as the ''stomach flu,'' although this is a common misconception. What people call stomach flu or “24-hour flu” more commonly turns out to be viral gastroenteritis.

Relevant Conditions
  • Secondary pneumonia
Common Types
  • Immunology based (Influenza A [H1N1], B, C)
  • Creature-of-origin based (swine flu, equine flu, avian flu)
  • Place-of-origin based (Shanghai flu, Taiwan flu, Denver flu)
Share this article
  • Centers for Disease Control (CDC) and Prevention. "Key Facts about Influenza (Flu) & Flu Vaccine." Feb. 2013. Accessed July 2013.
  • The Cleveland Clinic. "Influenza." Diseases & Conditions. Jan. 2013. Accessed July 2013.
  • CDC. "Types of Influenza Viruses.” March 2012. Accessed July 2013.
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