Most of the flu virus circulating is the H1N1 strain, which in 2009 caused the first flu pandemic in the US in more than 40 years. The strain tends to affect children and young, middle-age adults. So far at least six children have died this season.
The highest rates of flu have been reported in Texas, Missouri, Mississippi and Alabama.
Government health officials say H1N1 is covered in the vaccine, so it's critical to get protected if you haven't already.
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