A 17-year-old boy who suddenly collapsed and died was discovered to have the flu despite showing no symptoms.
Jonah Smith's heart stopped beating in the back of his sister's car last month, causing him to become brain dead from lack of oxygen.
Doctors confirmed the teen from Arizona tested positive for the flu and pneumonia after his death, though he did not show any of the typical flu-like symptoms such as a fever, sneezing or coughing.
Jonah's family is still grappling with his mysterious death that has made him one of the 20 children killed by the flu that is now a nation-wide epidemic across the US this season.
Jonah Smith, 17, collapsed and died last month when his heart stopped beating in the backseat of his sister's car
After his death, doctors confirmed he had the flu and pneumonia however his family said he showed no signs of being sick
Doctors suspected that Jonah may have had an undiagnosed heart condition, however toxicology tests determined that was not the case.
The only illnesses doctors can confirm are influenza 1a and pneumonia.
Jonah's mother Gayle told FOX Phoenix: 'I've already been told we may never know. The medical examiner results won't be back for at least six months.'
On December 29 Jonah's sister picked him up from work at a fast food restaurant before he collapsed in the backseat of her car.
His sister said that Jonah told her he was feeling sick minutes before his heart stopped beating - the only warning sign that her brother was suffering any illness.
'He didn't look like he had a cold or act sick,' said Gayle.
The only sign the family can pinpoint is that he complained of a backache, which did not stop him from going to work or seeing friends.
Jonah was airlifted to Phoenix Children's Hospital where he was declared brain dead due to lack of oxygen.HOW CAN THE FLU BE DEADLY?
The flu often leaves sufferers feeling sick and riddled with a fever, chills, aches and cough.
However, in extreme cases it can be deadly.
The flu poses the biggest risks to the elderly, young children, pregnant women and those with a compromised immune system.
Asthma, diabetes and heart disease patients also face an increased risk of death from the usually harmless virus.
But the flu, which has so far claimed the lives on 20 children and 85 adults in the US, can be life-threatening, or cause potentially fatal complications.
It can lead to pneumonia – but could also trigger sepsis – when an infection such as blood poisoning sparks a violent immune response in which the body attacks its own organs.
He died shortly after.
While most people suffering from the flu experience fever, chills, muscle aches, cough, congestion, runny nose, headaches and fatigue, not all those infected show symptoms.
Dr Angela Tonozzi director of Infectious Disease and Prevention at Aurora Health Care told Daily Mail Online that showing no symptoms is called the flu subclinical disease.
She said: 'Subclinical disease is known as carrier status because you have the virus and are able to share those germs without symptoms.
'You can have the flu but your body fights it and you have no knowledge that you had the disease.'
Typically someone would show the signs of the flu before it gets to the point of death, leading experts to believe that Jonah may have had a previous medical condition.
His death comes as this season's massive flu outbreak is devastating the US.
This season the flu has so far killed 20 children and more than 85 adults in the US, according to the CDC.
The rate of hospitalizations almost doubled in a week, leaving hospitals overcrowded, understaffed and turning some patients away.
Experts warn that infants and elderly people are the most vulnerable and the deadly virus has left an eight-month-old boy fighting for his life in Texas.
The flu is now widespread in more than 46 states as the outbreak reaches his peak, making it one of the worse flu seasons in years.