Five years ago Simon Dedrick was working as a family doctor in Bradford. Fit and active, in his spare time he refereed rugby matches.

Today his life is very different. He is no longer a GP, nor does he race up and down the rugby pitch.

Simon has been forced to dramatically change his life after being diagnosed with a condition that affects one in ten million people, and of which there are only between 300 and 400 cases in the world, none in Yorkshire.

Simon, 49, has Erdheim-Chester disease, a rare condition that can affect many different organs of the body.

It has left him with limited mobility, balance problems, respiratory difficulties, problems with vision, and other health concerns.

But he has not let the rarity and seriousness of the disorder stand in his way and is determined to live life to the full.

And he credits a weekly, hour-long exercise class held near his Queensbury home with helping him to improve his mobility.

Upbeat and cheerful, the former doctor for Bradford Bulls jokes: “I spent 20 years telling people that they should not worry, they have not got a rare illness, just a bad cold, and then I get something that only three or four people in the country have got.”

The symptoms first appeared a decade ago. “I noticed that something was wrong while walking the dog,” he says. “It was as if I’d had a few beers. I was a bit slurry and unsteady. I thought I had the beginnings of multiple sclerosis.”

Alarmingly, that “slight wobbliness” was soon accompanied by double vison, impacting upon his ability to referee at local rugby league games.

Tests for a possible brain tumour came back normal. Further tests followed and he was not finally diagnosed until four years later at Bradford Royal Infirmary. “I’d had one day off work in 18 years, then I was off for six months in a period of four years including three weeks in an isolation ward,” he says. “I was on a ventilator for two weeks and on steroids for six months.”

Erdheim-Chester disease is characterised by the over-production and accumulation of specific cells whose normal function is to fight infections.

These cells, called histocytes, accumulate in the loose connective tissue of the body. As a result, this tissue becomes thickened, dense and fibrotic. Multiple organs can be affected and unless successfully treated, organ failure can result. There is no cure, but treatments can control and sometimes shrink the growths associated with the condition.

“Many people die within three years of being diagnosed as they have become so ill before the final diagnosis is reached. Their organs fail and cells grow abnormally,” says Simon.

The disease – first described in 1930 by the Austrian pathologist Jakob Erdheim and American pathologist William Chester – can affect anyone at any age, although it is more common in men aged over 50.

Simon, who lives with Jo, his radiographer wife of 11 years, has a treatment programme. “It is a form of chemotherapy once a week that suppresses the growths, and I also take other medication,” he says.

He also attends a weekly Extend class after spotting an advert for it in the paper. “I read that it was for the over-60s and people with mobility problems, so I rang to ask whether I could be involved,” he says.

The class follow exercises to music while seated, standing and supported. Tutor Sarah Warner is thrilled by Simon’s progress. “Since he joined the class I’ve noticed that he has been able to do more standing up.”

The class is geared to those who, because of their age, condition or disability, cannot go to exercise classes or to the gym. “We use batons and bands – it is great fun. It is good for fine motor skills.”

She adds: “There is around 40 years difference between Simon and the oldest person in the class. I vary the music so there is always something everyone likes. People do as much or as little as they are comfortable with.”

Says Simon: “When you are not well you adapt your life to what you can do. To most people standing on one leg is not a limit, but it is for me. The Extend class is also about making your brain work – like changing direction and co-ordinating movements.

“I used to empty the dishwasher using one hand, while leaning on the side with the other. Now I can do it using two.”

It also helps him to play the trumpet at home.

Erdheim-Chester disease causes neurological damage leading to ataxia, a condition which affects the parts of the nervous system affecting balance and movement.

Witnessed by anyone unaware of this, it could lead them to think that the sufferer has had one too many. Simon delivers the correct message with good humour, wearing a T-shirt reading: ‘I’m not drunk. I have ataxia.’ He cannot drive, but gets about locally on his ‘Harley’, a motorised scooter with a maximum speed of 8mph.

Having had a busy, satisfying career, life is very different. “One of the biggest changes is not meeting so many people. I used to see up to 40 people every day , now when my wife - who has been a great support to me - goes to work I’m by myself. It’s a big thing when the postman comes.”

He still sees former colleagues from his last practice in Low Moor and is an honorary member of their ‘ladies’ who lunch’ club.

“There is no point in being negative. It was not in my life plan to end up like this, but I don’t sit outside worrying all the time.”

* Sandra’s Extend class is held on Tuesdays at Victoria Hall, Queensbury, from 2.30pm to 3.30pm. For more information, ring (01274) 883978.