The U.S. Centers for Disease Control and Prevention monitor and report on influenza through the Influenza-like Illness Surveillance Network (ILINet). In addition, CDC staff collect data from state health departments report. The maps below show data from the state-level reporting.
Influenza activity in early December...
... and at the end of December.
The maps are based on:
...the estimated level of geographic spread of influenza activity in their states each week through the State and Territorial Epidemiologists Reports. States report geographic spread of influenza activity as no activity, sporadic, local, regional, or widespread. These levels are defined as follows:According to the most recent information, flu virus is active in every one of the contiguous states and in Alaska. Hawaii is reporting only sporadic flu activity.
No Activity: No laboratory-confirmed cases of influenza and no reported increase in the number of cases of ILI.
Sporadic: Small numbers of laboratory-confirmed influenza cases or a single laboratory-confirmed influenza outbreak has been reported, but there is no increase in cases of ILI.
Local: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of the state.
Regional: Outbreaks of influenza or increases in ILI and recent laboratory confirmed influenza in at least two but less than half the regions of the state with recent laboratory evidence of influenza in those regions.
Widespread: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of the state with recent laboratory evidence of influenza in the state.
CDC cautions, however, that the maps reflect the "geographic spread of influenza viruses, but does not measure the severity of influenza activity."
For more information CDC also provides an interactive mapping tool, Flu View.