James Dodd, who attends nursery at Hobletts Manor Infants School, was born a healthy, happy baby but a severe bout of tonsillitis when he was just three caused him to have a seizure. This triggered off inside him the symptoms of the rare condition mitochondrial disease.

His mum Angela Barton, of Hemel Hempstead, says his mitochondria – the ‘batteries’ which power all of his cells – don’t function properly. He needs to take a cocktail of vitamins three times a day.

She said: “Most people do not know about the disease, it is very rare. It isn’t a good outlook for James, so we don’t know how long we have with him. No-one can tell us.

“He knows he is a special boy, but I don’t think he really understands it. I don’t want to go into it too deeply with him because I don’t want him to become scared of it.”

Although there is no cure and no other treatment available for James, those around him are completing two sponsored bike rides for the Lily Foundation, which not only provides support and bereavement services for sufferers’ families, but also funds research into finding a cure for the little-known killer.

Twenty-four of the family’s closest friends and relatives, including James’ dad Iain, will be cycling from London to Brighton in June, and again in September. They have already beaten their target, raising more than £2,000.

Angela, who gave up work in order to make the most of her time with James, said: “In a sense, the races aren’t for James, because we know nothing can help him. But the least we can do is put our time and money into finding a cure which could one day help someone else.”