Influenza (“flu”) is a contagious viral infection mainly
affecting the nose, throat, airways, and lungs. It is one of the
most changeable viruses; changes in the proteins of the virus
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, more than 36 000 deaths and 200 000
hospitalizations are related to seasonal influenza each year.
The October 19, 2011, issue of JAMA includes an article about
H1N1 influenza. This Patient Page is based on one previously
published in the November 4, 2009, issue of JAMA.