"Seasonal influenza epidemics recur yearly due to subtypes of influenza that circulate worldwide. These epidemics are responsible for an average of 36,000 deaths annually in the United States. Seasonal influenza primarily impacts those in the community with weaker immune responses (the very young, old and chronically ill) since most people develop some degree of immunity to the viruses through annual illness or vaccine. This immune response helps protect from the serious consequences of influenza. Influenza pandemics, however, are distinct from seasonal influenza epidemics and represent one of the greatest potential threats to the public's health. Pandemic influenza refers to a worldwide epidemic due to a new, dramatically different strain of influenza virus. A pandemic virus strain can spread rapidly from person to person and, if severe, can cause high levels of disease and death around the world."
2007 King County, Washington
King County, WA: http://www.kingcounty.gov/