ELIZABETH Quigley has just returned from a cycling holiday with her husband in the Lake District. There, on two wheels, with her weight off her legs, she found she could cover good ground with poise and balance. But, here, in the Tun bar, metres from the Edinburgh BBC offices where she works as a reporter, her feet carry her less smoothly across the floor. Without a stick she struggles to walk 100 metres. This means that the Scottish Parliament, just a short stroll around the corner, is beyond her reach. The local newsagent, a block up the road, is also too far. "If I ran out of milk, I wouldn't walk the distance to that shop to get it. I could if I used my stick, but it involves me accepting using the stick and I don't like it. I feel I don't want to be labelled by using it. Yet, how stupid is that? If I stagger down the street and fall over, what kind of label am I going to get?"

To most of us, 100 metres is nothing. But in the eight years since she was diagnosed with Multiple Sclerosis, Elizabeth Quigley has seen the scope of her physical world shrink. The stage and form of her disease is described as "relapsing-remitting" (often, but not always, a precursor to the more severe "progressive MS"). This means she experiences relapses in which old symptoms recur or new ones surface, interspersed with periods of partial recovery. She has, she says, had quite a few relapses since her diagnosis, though she is not sure exactly how many. "They vary between attacks involving a problem with walking, or a tingling face and weakness in my hand, and a full relapse which takes longer to bounce back from."

Multiple Sclerosis is caused by the destruction of an insulating layer called myelin around the nerves, resulting in a poor signal being carried from the brain to the limbs. It is as if, says Quigley, "the messages aren't quite getting through". The problem is worse on one side, and she struggles to lift her left foot clear of the ground.

Quigley first talked publicly about her illness in an article in this newspaper, just over a year ago. Before then, only a small circle of people, including her husband, Scotland's current finance minister John Swinney, knew of it.

The piece was her way of breaking it to many others. Before publication, she worried that readers might think she was saying: "Look at me!" She also doubted that it would be of interest or help to anyone. But the response to her article transformed her view of the power of personal journalism. One reader emailed to say that she had used the piece to break the news of her own MS diagnosis to someone, handing the magazine over and saying: "Read this. I've got this." The feedback, she says, was humbling.

Quigley hesitates to say that her condition has deteriorated since then. "I don't want to say it's getting worse, but gradually it has " she falters. "I am a bit worse than when I wrote the article. But I don't like to say that, because I have good days and bad days." "Worse" is not a word that comes easily to her. She is a resolute positive-thinker.

Quigley's daily life is coloured by small struggles. Unlike most of us, she has to think about the act of walking. If she doesn't, or if she gets up too quickly, she risks falling over in the first few steps. Getting up from her chair now, she demonstrates how she consciously brings herself into the process of walking, tucking her pelvis under before moving off. The resulting stride is awkward, as if her hips are lagging behind her perfectly postured torso. Pilates, she says, has helped her cope with the disease, strengthening crucial core muscles.

Mostly however, she finds ways around having to walk too far. BBC cameramen now go out of their way to drop her off at the interview location, rather than forcing her to walk from the parking spot. When she is out with her husband, he will often carefully rearrange the world around her. "He's very good at making it easy for me," she says. "He'll say, We'll just sit down here' or Why don't we all sit down here?' rather than asking if I want to sit down." She rarely goes to the supermarket. Shopping is done by Swinney, or ordered online.

If tiredness afflicts her, she shows no sign of it. Though seated, she is constantly in motion, hands animated. In her Sunday Herald article, she wrote that her doctor's main advice had been "to avoid stress and not get too tired or too warm". Yet, stress and tiredness are knitted into her life. "My medical consultant must despair of me," she wails. "But look at it this way. It might be extremely stressful for me to not be working at this job. Some people think if I have a quiet day spent sitting down, that's much better for me. Not really. If I sit the whole day then I'll stand up and my legs won't work."

Quigley is here to discuss a documentary she has made for the BBC, which deals with a question that has troubled her over recent years. Scotland has the highest MS rate in the world: why? Quigley had found herself being repeatedly asked for her views on this question, and as a journalist she found it frustrating that she had no answer. The documentary travels to Canada and Orkney to look at the many factors that increase an individuals chances of getting MS. But for Quigley, this is also a very personal journey.

Having intended her Sunday Herald article to be her sole public discussion of her disease (she didn't want to be a "poster girl" for MS) Quigley initially balked at the idea of the documentary. "I was worried about putting so much that is personal into the public domain. I'm quite private. And yet, I did think the subject was important."

What she discovered in the course of her journey is that there is no single answer to the question of why the MS rate in Scotland is so high. There appears to be a genetic element. Orkney, for instance has the highest levels in the world - it has its own MS society - and scientists are showing that it is passed down through families there. It also appears MS may be a disease we have exported to other countries. In Canada, for instance, MS is particularly high in areas to which Scots emigrated. What does this mean for Quigley, a Glaswegian? She has no Orcadian roots nor does she know of any member of her family who has had the disease. Yet in genetic terms, it may be enough that she is Scottish.

Our lack of sunlight may also be a factor. MS has been associated with low levels of Vitamin D, and when Quigley was tested, her levels were found to be low. There may also be a link between a childhood virus and the disease. Using herself as a case study, Quigley asks: "Is it because I'm Scottish? Maybe. But you've got to have something else. Is it because I had glandular fever when I was young? But many Scots have viruses. Is it because of low sunlight? Maybe. But again there's got to be something else."

While the programme does not provide any ultimate answers, suggesting rather that MS is the result of a complex series of events and conditions, it does provide hope. During filming, Quigley met Professor Charles Ffrench Constant, the head of the Edinburgh University Centre for Translational Research, who thinks a cure will be possible in the not too distant future. Drugs, he believes, could be developed to reactivate the brain's own stem cells into producing myelin and repairing the nerve system itself. To Quigley this sounded "like science fiction". But, she points out: "He talked in such confident terms. And I've interviewed quite a few scientists. They never leap to the golden conclusion."

Quigley is not prone to feeling sorry for herself. "Why me?" is not a question she has ever asked, at least not in a "woe is me" kind of way. The question posed by her programme addresses is not "why me?" but "why here?". Yet as she talks about our "beautiful country" - the one that produced this disease and sent it round the world - I can't help thinking: what about this beautiful young woman and the fact that she has that disease? Might "why here?" actually be a distancing question: a way of approaching the "why me?" issue, without facing it straight on.

"A-ha. I suppose," she says thoughtfully, when I put this to her. "Distancing? A-ha." Quigley takes a disciplined approach to her illness. Every evening, she injects the drug, Copaxone. Does it have any effect? "I don't know," she shrugs. "At this stage I'm not aware of it making a difference, but it's early days." She swims twice a week, in the mornings, before work. She does physiotherapy exercises every night. She is teetotal. "I can walk down the street and fall over without alcohol," she says. "Why pay to get something you've got already?"

She recalls, some time after her diagnosis, reading a book called When The Road Turns by Margot Russell. As she paraphrases its opening lines, tears well in her eyes: "Where were you when the road you were on in your life turned? Were you just starting out in life? Had you already had a family?" It's not difficult to see why this moves her. When Quigley was diagnosed she was 28 years old, with a bright future as a political journalist ahead. That word, "turned", matters to her. She was determined that this was not the end of any road for her.

In fact, Quigley has veered very little off her original road. Asked if her career would have been very different had she not developed the disease, she says: "No." Rather, what has had the biggest impact on the course of her career, is not the disease but love. When she married Swinney, then leader of the SNP, she gave up political reporting, her big journalistic love, to avoid any conflict of interest. Working now as a general reporter she sees no limits to what she can do. "I don't run round the block but then I wasn't very good at that anyway."

At 36, Quigley doesn't have children, although Swinney has two from a previous marriage. Has MS had any impact on whether she decides to have children? "I think, given MS is also triggered by those other factors, the risks of passing it on genetically are actually very slight. I don't think that would be a factor in a decision about having a family."

Might the question of her own health be a factor? "Some women," she points out, "who have MS, suddenly find when they get pregnant all their symptoms disappear. You hear of MS sufferers who have lots of children because they feel so well."

Often, when Quigley is explaining her condition to other people, she will emphasise it is not going to kill her. "You spend a lot of time telling people you're not going to die," she says. However, although no-one dies directly of MS, there are those whose suffering is immense. Quigley doesn't look in those very dark places. She and her husband do not discuss how they would deal with the illness if she got much worse. "Not at all," she says. "I don't think about that. I can't worry about something that might never happen. That would make things worse for me."

People often ask her what's going to happen next. "It's difficult for them to grasp that I don't know. And that I don't mind that. I just take it as it comes." She also sees no point in looking at other people's experiences when MS follows a different path in every person. "I know people at all different stages and it is so different for everybody. So, when I see somebody who is having, say, problems speaking, I don't think these things are going to happen to me." Her finger plots a graph in the air of her own pattern of disease, a steady slight downward slope with the occasional small dip. There are some sufferers, she explains, who have one attack and then have no further symptoms. Others go into steep decline.

While filming her documentary, she visited an Orkney MS sufferer, who, though just a year older than Quigley, was confined to a wheelchair. After the interview the woman asked her how she had felt about coming to visit her. Quigley, in characteristic journalistic fashion, turned the question on her: how did she feel? "Guilty," the woman said.

"Why?" Quigley asked.

"Because seeing me like this, you might think this is what's going to happen to you. And it might never happen."

"Well, I feel guilty too," Quigley countered. "I felt guilty coming to see you because I walked in here."

Quigley sees her disease as a journey, a "mystery tour". What is the next stop? How does she see her future? Very much, it seems, as the present. She hopes her current graph of mild deterioration will remain the pattern, or that the disease might level out, but she does not entertain thoughts about getting better. She is cautious about entertaining the hope that Ffrench-Constant's hoped-for cure might help her. "I'm not holding out for it. I don't think it will be for me and I'm not letting that thought in. But maybe there will be a cure for other people."

Then, once again, she switches from the subject of "me" to her favoured "here". "We've got the highest rate or MS in Scotland," she says, brightly. "Maybe we'll find the cure. There's a man not very far from here who thinks maybe we will. That's good. It's turning a negative into a positive."

BBC Scotland Investigates: Scotland's Hidden Epidemic - The Truth About MS is on BBC1 this Wednesday, 10.40pm