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.