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. Centers for Disease Control and Prevention 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.
