For a boy of 12 who is allergic to food and drink, Ben McGrath looks remarkably well. He has never come close to managing a full meal in his life, spending all of it fighting an intolerable disease that makes his body recognise food as an infection and so repels it.

From the day he was born, Ben's mother Jocelyn knew something was wrong. His sisters, Louise, 14, and Carly, 17, both had milk intolerances as babies and doctors suspected that Ben suffered from the same problem. But Jocelyn was unconvinced.

"Ben was always agitated, he was thrashing about and in a lot of pain," she said. "His first Christmas was spent in Bedford hospital, where we rushed at 6am after he passed blood from bleeding internally. The doctors there didn't really know what was wrong with him. One suggestion was tonsillitis, but I knew that wasn't it. It got to the stage that whenever I'd feed him he would scream, his body would spasm and he'd be in pain for hours."

Ben couldn't be given any form of baby milk without some reaction, so he was again admitted to hospital. Weekly visits followed for many months, with Ben reacting violently to anything he was given. "We gave him a piece of banana once, and he screamed and spasmed for 24 hours. I held him in my arms, rocking him to try to get the spasms to stop," Jocelyn said.

Doctors in Bedford were unable to diagnose what was wrong with Ben, who was then transferred to Great Ormond Street hospital in London. At 18 months Ben was finally diagnosed with eosinophilic enterocolitis allergic enteropathy, where the body mistakes food for an infection. In Ben's case, it causes his body to spasm violently and bleed internally. He is one of only a handful of known cases in the country. Since diagnosis, Ben has been on large amounts of medication to rectify the internal damage done to his body in the early stage of his life.

For more than 10 years he has been fed a special elemental formula in a one-litre feeding pack via a tube, which is inserted directly into his stomach overnight, giving him 70% of the medication and nutrients he needs.

But Jocelyn and her husband Walter, from Great Barford in Bedfordshire, still have a special arrangement with their local hospital in the event of yet another emergency. "About two years ago Ben's body just collapsed," said Jocelyn. "He was unconscious in hospital until the doctors managed to bring him round. On his birthday that year he was rushed to Great Ormond Street for an emergency operation after his feed burst. The doctors have been superb with us.

"Every time we take Ben to Great Ormond Street, they say they will get Ben through this, but they don't know exactly when. It's been horrendous for us as parents and hard on his sisters, too. They've been through hell with him, and they are so protective."

The McGraths have been consumed by Ben's condition, coming home early from holidays or cancelling them altogether if necessary. But a new therapy under trial at Great Ormond Street has given Ben hope that he might be able to have the tube removed when he is 15 and live a normal life.

"The children who have variants of this disease all react in different ways to treatment," said his mother. "We are hopeful, though. All Ben wants is to do things that other normal children his age do. I have told him he'll be able to have a pint of beer when he's 18, he just has to be patient. He gets frustrated at times and has occasionally crammed as much sweets and chocolate into his mouth as possible just for the hell of it, knowing how poorly it will make him."

Ben can now eat some foods, like fruit and bread, but only in small quantities, and he still needs to be connected to a feeding pump for 10 hours a night.

His father said: "He can eat other things like ice-cream, but there is a price to pay and his body will have a reaction, and he knows that. Some foods make him quite aggressive, others drain his energy. Christmas can be horrible for him because he just wants to eat what his friends eat, and he can't."

Despite falling ill before Christmas, and being unable to eat anything for three days, Ben enjoyed the festive season, according to his mother.

"If you saw Ben, you wouldn't think there was a single thing wrong with him. He is doing well now, but it has been a constant battle to get him to this stage. We've always encouraged Ben to lead a normal life and tried to make life work around the illness," Jocelyn said.

"He looked at me the other day and said, 'I didn't eat too badly over Christmas, did I, mum?' We just look forward to the day he can eat anything he likes."