(CNN)If it seems like you know a lot of people who've come down with the flu lately, you're on to something.
Nineteen states now have high levels of flu activity, according to numbers released Friday by the US Centers for Disease Control and Prevention. That's more than double the number of states from the previous week.
The number of children who died also increased. Two children's deaths related to the flu were reported in the week ending December 29, bringing the flu's death toll for children to 13 for this season, according to the CDC.
Crystal White Shield knows the toll flu can take.
She lost her 6-year-old daughter, Allison Eaglespeaker, to flu on December 1.
Allison, a kindergartner in Missoula, Montana, loved to dance and to hang out with her big brother, Matt, and her little sister, Daisy.
Allison Eaglespeaker, 6, died of flu on December 1.
"She was a beautiful little girl," her mother said. "We were blessed with her life, and we miss her dearly."
Allison's mother wants other parents to be aware of how quickly flu can take a child's life. Her daughter died less than 48 hours after she first had symptoms of the flu.
"This illness hit her really hard and really fast," she said. "I didn't know I was going to lose my baby."
Dr. William Schaffner, who studies the flu, says that's not unusual; when flu kills a child, it often happens quickly.
He said it's not known why flu is a relatively mild illness for the vast majority of children and deadly to only a small percentage. Last year, flu killed 185 children, the highest number of pediatric flu deaths in a regular flu season, according to the CDC.
Allison had mild asthma, but about half the children who die from the flu were healthy before flu hit, according to the CDC.
This year's predominant strain, H1N1, also known as swine flu, tends to disproportionately affect children and adults under 50, said Schaffner, a professor of preventive medicine at Vanderbilt University Medical Center.
He said that's because similar strains were circulating about 40 to 50 years ago.
"Older people may have been infected with cousins of H1N1 years ago, and that gives them residual protection," Schaffner said.
The H1N1 strain tends to cause milder illness than H3N2, the predominant strain during last year's particularly severe flu season, and vaccination tends to be more effective against H1N1 than H3N2.
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Even though the flu season is underway, there are still potentially several months of the season left, and it's not too late to get a flu shot. The vaccine is recommended for everyone except infants under the age of 6 months.
Although the vaccine won't prevent all cases of the flu, it lessens the severity and duration of symptoms, and those who get flu after receiving a vaccine are less likely to require hospitalization and less likely to die.
Of the 185 children who died from the flu last year, 80% had not received a flu shot, according to the CDC.
Once someone gets the flu, antiviral drugs can lessen symptoms and hasten recovery, and reduce complications and hospitalizations. The drugs work best when started soon after someone gets sick.
