KUSA - The Colorado Department of Health says 748 people in Colorado have been hospitalized since January 2013, and that number has recently been growing every week.
Doctors say people aren't getting the flu shot, and many people aren't staying home when they have the flu, so it's spreading very quickly.
Colorado is seeing high activity of the flu, and many middle-aged adults are seeing the worst of it.
The Department of Health says adults ages 25 to 64 are getting hit the worst - making up a little over 400 of the 748 hospitalized in Colorado in the past year.
9NEWS Medical Expert Dr. John Torres says the 2013-2014 flu season is more severe than the 2012-2013 season.
The cases he is seeing in his clinic are the H1N1 strain, and the good news is the vaccine covers it.
Colorado lab tests confirm that a majority of cases are the H1N1 - the same swine flu seen in 2009.
Children's Hospital of Colorado says they have hospitalized more than 70 children since October 2013, and most of those kids were not vaccinated.
In Colorado, there have not been any pediatric deaths so far this flu season.
Denver Health says they have had 76 flu hospitalizations this flu season, and of those, only 16 people were vaccinated. They recommend getting the shot.
They say it takes two weeks for that shot to kick in, but they say symptoms will be less even if you do end up getting the flu.
The flu shot is still available. Rumors have been circling that the vaccine and the treatment Tamiflu are running out, but Dr. John says they are plenty left.
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