Alternative Names

Influenza A; Influenza B

Definition of Flu

The flu is an infection of the nose, throat, and lungs caused by the influenza virus.

Causes, incidence, and risk factors

Influenza A usually arrives between early winter and early spring. Influenza B can appear at any time of the year. H1N1, or swine flu, is another type of influenza.


The flu usually begins quickly. The first symptoms are a between 102 and 106 °F. (An adult typically has a lower fever than a child.)

Signs and tests

Most people of do not need to see a doctor or nurse when they have flu symptoms. This is because most people are not at risk for a severe case.



Expectations (prognosis)

Millions of people in the United States get the flu each year. Most get better within a week or two.


David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc. – 10/5/2010

Normal lung anatomy
Nasal spray flu vaccine

ADAM Medical Encyclopedia

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