Influenza surveillance data sets are readily available from the Centers for Disease Control and Prevention (CDC) website at www.cdc.gov/ncidod/diseases/flu/. They include the following:
• Weekly state epidemiologist reports of influenza activity.
• Weekly regional number of influenza-like illness2 (ILI) cases from the U.S. Influenza Sentinel Physicians Surveillance Network.
• Weekly regional number of positive influenza tests.
State health department estimates are a second source of influenza data. State health departments report the estimated level of influenza activity in their states each week using four levels:
• No Activity: No cases of influenza or influenza-like illnesses (ILI) reported.
• Sporadic: Cases of influenza or ILI are reported, but reports of outbreaks in places such as schools, nursing homes, and other institutional settings have not been received.
• Regional: Outbreaks of influenza or ILI are occurring in geographic areas containing less that 50% of the state's population. A geographic area could be a city, county, or district.
• Widespread: Outbreaks are occurring in geographic areas representing more than 50% of the state's population.
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