Health authorities say flu activity is widespread in Pennsylvania, meaning it's affecting at least half the regions of the state.
More than 5,000 people have fallen ill as of Dec. 20.
Allegheny County has the most confirmed cases, with over 800.
However, officials say these figures represent only a fraction of likely cases. Many more people don't seek medical attention or lab testing to confirm the virus.
Flu season typically runs from October through May. The health department estimates between 5 percent and 20 percent of Pennsylvanians get sick each year.
Officials say about 120 to 2,000 state residents die annually from flu-related complications. Eleven people have died so far this season.
One Chester County teen, who died Dec. 19, tested postive for influenza A. Health officials have yet to confirm his cause of death, although family members said complications from the flu contributed to his passing.
