This year's infecting virus is the H1N1 strain that emerged in 2009.
In some states, the virus is producing severe respiratory illness, mostly with young and middle-aged adults.
Several states are dealing with a high number of cases, including some deaths.
Here in Michigan, estimated flu activity has been upgraded from local to regional activity.
That's due to a recent rise in cases in soutwhestern and central Michigan.
Victor Blackwell has a closer look at how other states are faring.