Influenza, also known as the  flu, is an infection of the respiratory system that is caused by the influenza virus. Influenza is spread by coughing and sneezing. Influenza season peaks between late December and March, although this can vary each year. The flu is responsible for an average of 226,000 hospitalizations and 36,000 deaths every year in the United States.

Click to open: 2009 H1N1 Flu