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Influenza ("flu") is a contagious viral infection mainly affecting the nose, throat, airways, and lungs. It is one of the most changeable viruses; changes can be small and continuous or abrupt. Flu outbreaks occur yearly, usually from November to March in the Northern Hemisphere. Approximately 5% to 20% of Americans are infected every flu season. Most people improve within a week, but for elderly people, infants and children, and people with some chronic diseases, influenza can be life-threatening. In the United States, seasonal influenza kills more than 36 000 people and hospitalizes 200 000 others yearly. The November 4, 2009, issue of JAMA includes several articles about influenza.

TYPES OF INFLUENZA

Influenza viruses are classified as type A, B, or C. Type A is the most common and causes most of the serious epidemics. It infects many animals, such as ducks, chickens, and pigs, as well as humans. Type B can cause epidemics . . . [Full Text of this Article]

Huan J. Chang, MD, MPH, Writer; Alison E. Burke, MA, Illustrator; Richard M. Glass, MD, Editor

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