The parents of a three-year-old with an illness so rare it affects just 27 others have recalled the terrifying moment they first knew something was wrong with their son.

The couple are reliving the ordeal in a bid to raise vital cash to fund research into the mysterious condition that affects their boy Angus, and just a handful of others. 

Experts are all-but in the dark about his genetic problem, which causes constant seizures, speech and language problems as well as movement issues.

Youngsters can also suffer heart defects — and some hit by them don't make it, sadly. 

And so this week dad Oli Powell, 34, has embarked on a remarkable charity mission, running seven gruelling full marathons — each 26.2 miles long — in seven days. 

He has already raised more than £30,000 for CRELD1 Warriors, which provides support to families as well as spearheading calls for better medical treatments. 

Former Welsh guardsman Oli admits the challenge has pushed him to the limit physically — yet he insists it's nothing compared to the struggles children affected by CRELD1, like Angus, and their families face on a daily basis. 

Mum Sophie, 35, was at home when she noticed Angus, then six months old, 'didn't seem to be feeling well'. 

Thinking he was simply under the weather, she lay him on the bed to give him Calpol, but within moments he began suffering a terrifying seizure. 

Sophie, fearing for his life, called an ambulance. 'His eyes were rolling back and his body was jerking — I honestly thought he was dying,' she admits. 

Although he was kept in hospital overnight, he recovered rapidly — and, frustratingly, doctors could offer no explanation for why it had happened. 

That was that start of an almost two-year ordeal during which Angus suffered repeated seizures, some hours-long, and had to be put into an induced coma — with tests unable to reveal the cause.

Six months ago, the family were finally given an answer: Angus has a condition called Cysteine Rich With EGF Like Domains 1, or CRELD1 for short.

It occurs in children when both parents pass on a vanishingly rare faulty gene — and, unknowingly, Oli and Sophie carried it. 

Fortunately, their younger daughter Lyla, who is 16 months, is unaffected.

Sophie is pregnant and testing has revealed the baby is also healthy, thankfully. 

A wide range of health issues related to CRELD1 have been identified, including heart problems, learning and developmental delay, as well as respiratory and immune system issues.

Sophie said: 'Before his first seizure there was no inkling Angus had a problem — he was a happy, healthy boy.

'That seizure lasted for about five minutes, but it felt like for ever. He was a bit sleepy afterwards, but seemed to recover.

'The doctors kept him in overnight and suggested it might just have been febrile convulsions [mild seizures that babies commonly suffer as a reaction to a fever].'

But over the months that followed, he was in and out of hospital, suffering severe colds that triggered further seizures that were ever worse and longer. 

'Still they weren't able to tell us why it was happening,' said Sophie. 'The doctors kept saying that he might grow out of it.'

Tests all came back normal, and the couple were offered genetic testing — but once again, the results threw up nothing. 

Last Christmas Eve, Angus suffered a seizure that lasted for hours — it was so bad, doctors decided to put him into a medically induced coma. This is to protect the brain when all other treatments have failed.

He pulled through and six months ago a call came from the John Radcliffe Hospital, Oxford, with an answer. 

'From our blood samples they'd finally worked out he had something called CRELD1 — the doctor said there was just one medical paper written on it so far, so they weren't sure how it would affect him or what would happen,' Sophie said.

The couple were put in touch with CRELD1 Warriors — a support group for the families affected by the condition. 

'We worked out he is probably the 28th person ever to be diagnosed,' said Sophie. 'We also realised that we were lucky, as Angus is only mildly affected. 

'Other children have multiple seizures a day, or seizures that last for up to seven hours.

'The daughter of the couple who founded CRELD1 Warriors sadly died from a seizure.

'Still, it's frightening not knowing how it will manifest in Angus.  

'We would love there to be more research, to find out exactly what's going on — and to find a treatment, because at the moment there is nothing.'

Despite his challenges, Angus is a normal little boy in many ways, says Sophie. 'He goes to nursery and loves animals, and running around outdoors.

'Communication can be tough as he has very little language but he can communicate if he's thirsty or hot, or needs something.

'He takes three kinds of epilepsy medicine every day, at the maximum dose.' 

Oli, who now works as a renewable energy expert, said: 'Getting in touch with CRELD1 Warriors was so important — to speak to other families who faced what we were going through. 

'I wanted to give something back and that's why I'm running these marathons.'

To sponsor Oli's seven marathons in seven days mission, visit the couple's JustGiving page.

He added: 'Seven marathons in seven days seemed like a nice idea, and I'd done seven half marathons and that was a doddle, so I thought this would be fine but I feel like I underestimated how hard it would be.'

His routes start at the family home in Chaddleworth, Berkshire, and he's been buoyed by support from army colleagues, friends, family and others in the CHELD1 Warrior community who have joined him on his runs. 

'It's been really difficult,' said Oli, 'but I'm going to finish it.'