By Katy Dartford
Tom died unexpectedly in his sleep from an undetected heart problem
They're fit and healthy with no idea they've got any health problems, but around 12 young people every week die from undiagnosed heart conditions.
That's about 50% higher than previously thought and anyone under-35 is at risk.
Claire Prosser's son, Tom, died suddenly a year ago.
She said: "He was an apparently healthy boy who was very lively, played a lot of sport, was very happy, then one day he simply died in his sleep."
Exercise doesn't cause these heart problems, but it can make any undetected problem worse.
"It's incredibly difficult to handle," said Claire. "You don't know where to start, you're in shock.
"We've stuck together very much as a family and we got involved with Tom's friends - it has been great to see them because they've all been suffering as much as we have been."
Tom's friend, Nick Scott, said he was really popular at school: "He was always making people laugh, he had loads of friends, everyone liked him.
"There wasn't anyone who didn't like Tom and he was just really nice all the time."
It was thought around eight people a week died from these heart conditions because they didn't show up in post-mortem examinations.
Many causes of death were being wrongly recorded as asthma, epilepsy or even drowning.
Alison Cox from the charity Cardiac Risk in the Young (CRY) is calling for health screening to be introduced in all schools.
She said: "In 50% of people there are no symptoms at all. When there are symptoms it's breathlessness, palpitations, dizziness and fainting."
Tom's friend, Michael Birse, was at his funeral and hopes that if anything his death will help save others.
He said: "We didn't know anything about it before what happened to Tom, so if we can raise awareness... hopefully we can come up with ways of stopping it."
Clare Prosser has kept a diary of how she's coped since Tom's death: "He was a fantastic kid who really lived life to the full, and I can hear him in my head all the time when I'm feeling miserable saying 'Come on mum, keep laughing', and that's what he would have wanted us to do."
You can hear Tom's mum's diary on BBC Radio 4 on 13 October at 2000 BST.