Until this month, 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. Often, it's half over by late February. CDC officials don't know why this season was slow but it could be a combination of factors.
On Friday, CDC officials said one state — 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.
