Influenza viruses usually circulate across the United States on a predictable, or “seasonal,” pattern with increased activity from October through May with cases typically peaking around February. However, in spring of 2009, a new influenza virus known as novel influenza A (H1N1) or “swine flu” began circulating. This new H1N1 virus continued to circulate at higher than usual rates (for flu) throughout the summer months of 2009, and because of its ease of transmission was declared a pandemic.

This Web site is specifically about seasonal influenza, but seasonal and H1N1 pandemic influenza viruses can cause severe illness and must be taken seriously.

More Information

For the most up-to-date information on novel influenza A (H1N1) infection, visit the Centers for Disease Control and Prevention’s dedicated Web site, www.cdc.gov/h1n1flu.