A 12-year-old boy in Florida, whose family say he was a healthy child, has become the latest American to die from an especially deadly form of influenza.
Dylan Winnick developed a fever on Monday and died on Tuesday, according to stunned family members.
More than 30 children have died this flu season - the worst since officials began tracking the deaths 13 years ago.
Experts say between 12,000 to 56,000 Americans die from the flu each year.
Most at risk are the elderly and young, officials say.
However, there have been many reported cases around the country of healthy people dying after displaying flu-like symptoms.
One of Dylan's family members told local media that he had no underlying health issues, and had attended a birthday party in the days before his death.
Mike Medwin, the partner of Dylan's mother, told CBS News that he had heard about the deadly flu spreading across the country, but never expected it to affect his family.
"You shrug your shoulders," Mr Medwin said.
"No way. We're healthy. The kid's playing soccer. He's strong," he added in disbelief.
Tragically, CDC reports 30 #flu-related deaths among children for the 2017-18 flu season. Parents and caregivers: here’s what you need to know to protect the children in your life. https://t.co/KtzLhlzIQ6 pic.twitter.com/MKdD9ZdfYy— CDC Flu (@CDCFlu) January 23, 2018
Experts at the US Centers for Disease Control and Prevention say H3N2 - otherwise known as the Aussie strain - is the most prevalent type of the flu virus spreading around the US this year.
A version of the virus also affects dogs, leading to vaccinations by pet owners. However, it cannot be transmitted between dogs and humans.
Schools in at least 12 states have closed this year due to outbreak of the deadly virus.
A school superintendent in Texas told Newsweek that its spread is "unlike anything we've seen before".
Jill Siler, from the Gunter Independent School District, told the magazine that the school closed for one week so that cleaning staff could come in and meticulously sanitise "every Lego and Potato Head" in a kindergarten classroom.
Some hospitals have been forced to erect overflow tents for the influx of patients.
Another recent victim is Katie Denise Oxley Thomas, a 40-year-old mother of three, who enjoyed running marathons.
"The doctor was saying that he had never seen anything like this, and it was not supposed to happen," her sister told the Mercury News.
Doctors say the peak of flu season may have already passed, but it will still be several months before cases dry up.
They add that it is not too late to get a flu vaccination, which is still the most effective form of prevention.