(CNN) - The flu is spreading in 40 states and Puerto Rico.
Health officials said that flu activity is "high" to "extremely high" in 15 regions.
The Centers for Disease Control and Prevention said 15 flu-associated pediatric deaths have been reported so far this season.
One expert says mid-Feb. is the peak, and it's not too late to get a flu vaccine.
Flu often starts with an abrupt onset of symptoms including fever, chills, headache and soreness.
Children, elderly and those with chronic conditions are at a higher risk of complications from the virus.