Kayla Linton was a healthy, all-around athlete, but being fit did not protect her from the flu.
Linton, who died last week in Baltimore, is among the dozens of often perfectly healthy children who die from influenza every year in the U.S.
It's shaping up to be an average flu season so far in the U.S. last year, but even an average flu season is deadly, the Centers for Disease Control and Prevention says.
CDC says 15 children under 18 have died in the 2016-2017 flu season. It's probably more than that — it takes a few weeks for CDC to gather the information, and not all states report flu deaths quickly or in the same way. In the last flu season, 89 children died.
Related: Flu Can Kill in a Flash
Linton, 17, was only sick for a few days.
"It's completely shocking," her older brother, William Linton, told NBC News.
"She's a perfect, healthy little girl. She played sports. She was involved with everything. It's just shocking that the flu could kill her like that."
Kayla did not have any especially troubling symptoms at first, William says, although their mother took her to a retail clinic and then to the hospital emergency room for a flu diagnosis.
"Later out through the week she wasn't getting any better. Then early Saturday morning she asked for help to go to the bathroom and when she got up she was having trouble breathing," William Linton said.
"Then, a few minutes later that's when she stopped breathing, and they couldn't do anything to bring her back when the ambulance picked her up."
Flu usually hits the very young and the very old the hardest. Depending on the season, it kills anywhere between 4,000 and 50,000 people a year in the United States. Because each flu case is not counted, public health experts have to estimate flu's toll.
Related: Flu Season Getting Worse, CDC Says
But every pediatric death is counted.
CDC says a child does not have to be frail or to have any underlying condition to become very ill from flu or to die.
"She was never sick. She played sports, three sports all year," William Linton said.
While the influenza vaccine is not always very effective, the CDC says it's the best protection against infection and against having a serious bout of flu. Doctors say it's still not too late to get one but urge people to get vaccinated in the fall if they can.
Right now influenza is spreading in all 50 states, and some communities are closing schools for a few days to control its spread. Rhea County schools in Tennessee just reopened after being closed Friday and Monday because so many students and teachers were out sick.