Until February, there weren't enough flu cases in the U.S. to signal the start of the season. This is the latest start to a flu season since the winter of 1987-1988.
Flu season usually starts in December or January and is half over by late February. CDC officials don't know why this season was slow.
Friday CDC officials said California has had widespread cases for the last two weeks, and Missouri has seen a spike in reports of flu-like illness.
So far the flu vaccine seems well-matched to the strains making people sick. Last flu season was considered average.
The Associated Press contributed to this report.