In the downstairs hall of Katrina and Mark Dujardin's elegant Cambridge house, there is a cupboard that would do justice to a professional pharmacy. From floor to ceiling, its shelves are lined with cellophane-wrapped boxes of antibiotics, jars of digestive enzymes, cartons of inhalers, phials of vitamin supplements and a myriad other medicines crucial to the struggle that absorbs the couple's daily life: keeping their daughter Anna alive.

Anna is 10 and suffers from cystic fibrosis, one of the most common lethal, inherited illnesses in the west. In Britain, almost 8,000 young people have this wasting metabolic disorder, which they inherit from symptomless, unsuspecting parents. There is no cure.

Despite her condition, Anna bustles with vitality. Dark-haired and petite, she has a bedroom - like so many girls of her age - that is a citadel of pink, with shelves filled with stuffed toy animals. She is chatty, likes to joke, and finds rich amusement in her doctors' constant interest in her spittle and other bodily fluids. "It's just weird," she says. In short, Anna seems the epitome of a healthy, fun-loving child.

Yet the battle to keep her alive has been hard. Without strict daily regimes of physiotherapy, and those drugs from the Dujardin medicine cupboard, sticky fluids would fill Anna's lungs and allow lethal bacteria to flourish. Thirty years ago such infections killed sufferers in infancy, and although antibiotics now allow patients to live to their thirties, the disease remains an intractable affliction. "No day passes without either of us thinking what lies ahead for Anna," says Katrina.

Tales like these are common among the thousands of other British homes touched by cystic fibrosis. The hardship brought by the disease is intense. However, a new sense of optimism has recently begun to spread through these families thanks to a unique British project: the UK Cystic Fibrosis Gene Therapy Consortium. Set up in 2001, with backing from patients, families and friends, it has assembled a group of leading scientists who are using advanced molecular technology to find ways to put healthy genes back inside patients' bodies. Treatments that might extend lives significantly may now only be a few years away, it is hoped.

It is an intriguing enterprise, right down to its financing - £33m raised by the UK Cystic Fibrosis Trust through its raffles, charity balls, coffee mornings, karaoke nights and sponsored overseas treks. As an exercise in patient power, the project is unprecedented.

Then there is the consortium's structure. Based in London, Edinburgh and Oxford, its 80 scientists account for virtually every one of the nation's geneticists working on treatments for cystic fibrosis. In effect, a national research institution has been created for dealing with one disease, and since 2001 its staff have worked tirelessly to reach its goal of creating gene therapy treatments for the condition.

And now those efforts are nearing fruition. Last month, the consortium began safety trials of its first product. A single dose of inhaled droplets carrying a healthy gene is being given to cystic fibrosis patients to see if they suffer harmful side-effects. "By June, we will have an idea if our treatments look safe. I am already hopeful," says Imperial College's Professor Eric Alton, the consortium's co-ordinator.

Then the second, key part of the project will proceed. A hundred individuals with cystic fibrosis will be recruited for a year's testing to determine not just that gene therapy is safe, but that it actually works and can halt the inexorable lung degeneration which ultimately kills 90 per cent of patients. "In a couple of years, we will either find out our idea was rubbish or experience a great yippee feeling," adds Alton. "I should stress that all our efforts have been directed to ensuring it will be a yippee." Cystic fibrosis patients might then be able to look forward to lives that extend into their 40s, 50s or beyond. For those already in their 30s, such a development cannot come soon enough - the erosion of their lungs can be relentless - while those in the early stages simply seek an end to the drugs, the constant lung-clearing exercises, and the prospect of a moderately long life. Much depends on the trial now taking place and on the patients selected for involvement, including the youngest recruit, Anna Dujardin.

Anna is Katrina and Mark's second child. "Our first, Jamie, was robust and healthy but Anna was clearly going to be much smaller," says Mark. "We didn't worry about that during the pregnancy. Then she was born and started to lose weight straightaway." Doctors deemed that she was "failing to thrive" before diagnosing cystic fibrosis two months later.

"Doctors are very straightforward," recalls Katrina. "They tell you, to your face, that the disease is terminal - a terrible word for a parent to hear. I felt as if a bus had hit me. I thought, if she has to go, could she go now because I couldn't stand to enjoy her and then have her taken away." For his part, Mark threw himself into his work (as a company director). "I was in denial," he admits. Since then, the Dujardins have fought hard to keep Anna from succumbing to her condition, a battle that reached its low point two years ago when she picked up an MRSA infection that brought her perilously close to death. She has since recovered, though the story illustrates the knife edge on which affected families must live.

Emma Lake, a 26-year-old cystic fibrosis patient, provides a different slant on this story. A keen ballroom dancer, along with her pharmacist husband Geoff, Emma lives in a tiny flat in Redhill, near Gatwick airport, which the couple share with a cat called Tart, "a shameless sponger for food". Emma's daily life is filled with a gruelling regime: two bouts of physiotherapy; handfuls of vitamin tablets to compensate for her system's inability to deal with fatty foods; between 50-70 digestive enzyme tablets a day; as well as the constant use of antibiotic and asthma inhalers. "It just takes all the spontaneity out of life," she says. "You can't go round to a friend's house, have a few glasses of wine, and then decide to spend the night. You have to go home to take all your medicines."

Even the solace of fellow patients' company is denied the cystic fibrosis sufferer because of the danger they pose to each other. Every patient possesses his or her own set of secondary infections, caused by their condition. When they meet, there is therefore a real danger of them exchanging these infections. "I had a Christmas job in a store and discovered another student there had cystic fibrosis," says Cara Doran, another patient. "We tried different shifts but it was impossible. One of us had to go, so I went." Cara is 30, lives on her own in Dunbarton, and works for the Cystic Fibrosis Trust as an adviser who helps the health service with other cystic fibrosis cases. "We recently had one young lad with the disease who clearly has issues with his condition, but I could not sit down and talk to him. The best I could do was phone or email. It is not the same."

Yet the cause of all these problems is such a tiny thing. Every human being has 23,000 genes coiled inside their chromosomes. These direct the construction of the proteins from which our bodies are made. But sometimes a gene contains a small error. In the case of one known as the CFTR gene (for cystic fibrosis trans-membrane conductance regulator), there exists a version which is missing three of its 250,000 units of DNA. This deletion causes the gene to make a misshapen protein which cannot carry out its key task: to control how chloride ions behave inside our bodies and to ensure that our cells produce healthy digestive juices, sweat and mucus. Instead, bodily fluids become thick, sticky and clog up lungs, digestive tracts and other organs. Sometimes enzymes from the pancreas often cannot reach the stomach, so patients cannot digest food. Supplements can help but often cystic fibrosis patients cannot obtain sufficient nourishment, and so tend to be smaller than average.

In addition, the sweat of cystic fibrosis patients is noticeably saltier than normal, a symptom whose significance was known long before cystic fibrosis was even recognised, in 1938, as a specific disease. "The child will soon die whose brow tastes salty when kissed," states one 19th-century European health almanac. On the other hand, this phenomenon also produces one of the disease's rare benefits, according to Emma Lake. "Like a lot of other people with cystic fibrosis, I don't get hangovers," she says. "I think it is because we sweat more than other people."

However, lungs remain the disease's main threat to health. Made of folded layers of tissue, a person's lungs would cover a tennis court if they were spread out flat, says Dr Chris Boyd, a leading member of the consortium's Edinburgh department. "Blood vessels go into tinier and tinier airways until they are only one cell layer thick. Here the oxygen molecules that we inhale are passed into the blood. This layer of cells is protected by a coating of mucosal liquid and also by tiny hair-like projections called cilia that beat upwards to waft away incoming objects, such as bacteria. But in cystic fibrosis patients, this liquid becomes thick and sticky and the cilia cannot beat properly and sweep away bacteria. Lungs quickly become infected."

Antibiotics can fight these bacteria, but infections still break out and damage accumulates. By the time they have reached their 30s, patients are often suffering major lung impairment. Hence scientists' decision to take their battle against cystic fibrosis to the lungs and to get healthy genes into cells so they start producing normal secretions. Then those cilia can beat properly and ward off bacteria. At least, that is the theory.

In fact, the pursuit of gene therapy for cystic fibrosis patients is already two decades old. In the 80s, scientists - armed with the latest tools of molecular biology - began hunting for the gene whose mutations are responsible for the disease. They succeeded in June 1989, when Lap-Chee Tsui from Toronto's Hospital for Sick Children pinpointed the guilty piece of DNA. "It was an epiphany," says Professor David Porteous, leader of the consortium's Edinburgh section. Discovering the CFTR gene and its mutations gave scientists the means to attack the disease with new vigour. Within months, a host of gene therapy projects were being put forward. Some proposed infecting patients' lung cells with a virus into which a healthy CFTR gene had been spliced. The virus would then carry the gene into cells where it would be incorporated into DNA and should start to make healthy CFTR protein. Other suggestions included making artificial chromosomes to house healthy genes inside patients' cells.

"I was only six when they announced they had found the gene but I remember my mum and dad being so excited," says Emma. "They thought gene therapy was round the corner. But then nothing happened." The trouble was that human evolution conspired against medicine. Patients' immune defences simply gobbled up those intruders and made short work of attempts to introduce healthy genes, says Porteous. "Our lungs are like Fort Knox. They have evolved defences that can repel everything we inhale, except oxygen."

This point is backed by Deborah Gill, who - with her husband Steve Hyde - leads the Oxford laboratory of the cystic fibrosis consortium. "In the end, we did manage to get genes into patients' cells and got them to work there, but not for long enough. The effect lasted days, not for the months you need for it to be an effective treatment." In the end, dozens of groups tried and failed, until by 2001, there were only three left in Britain: at Oxford, Edinburgh and London. And that is when Rosie Barnes, head of the Cystic Fibrosis Trust, intervened. Her charity raises money to provide support and money for care workers, clinicians, researchers and patients. "Rosie said that enough was enough and told us we had to pool our resources and co-operate on a single project, and that if we did we had a real chance of dealing with cystic fibrosis once and for all. She also promised to give us the money we needed," adds Gill. "Effectively she nationalised us."

But committing the cash was controversial. Essentially, the trust was moving its resources from providing day-to-day care for its members to carrying out a one-off gamble in the hope that it might revolutionise their long-term prospects. "There was blood on the carpet," admits Barnes. "Until then, we were providing considerable amounts of money for clinicians to help treat cystic fibrosis patients. We decided to stop that and commit nearly everything to gene therapy. It caused an almighty row. We were told that this could have devastating consequences for patients. However, the NHS - which should have been supplying that money in the first place - has now stepped in, leaving us to concentrate on the consortium project."

And now its scientists are approaching their goal: an artificial gene, a slice of DNA containing key pieces of a healthy CFTR gene coated in a fatty material - known as a liposome - to help it sneak past patients' immune systems. These will be inhaled as aerosol droplets - in sessions that will last for several hours - so that when a liposome brushes against a lung cell it will dissolve on to its membrane and its cargo, that tiny sliver of DNA, will slip inside the cell. "It sounds simple," says Steve Hyde, "but it has taken 80 researchers working flat out for eight years to design and make these droplets, and to show in mice that they can avoid immune defences and deliver healthy protein to lungs for months rather than days."

Just getting the gene into the right configuration for coating with liposome took five researchers a total of five years of cutting and pasting tens of thousands of DNA snippets, until they got it right. Even then, there may be unexpected setbacks or immune reactions. So alternative gene variants and coatings are being designed. "This is something Steve and I have worked on for 20 years already and it will see us through to the end of our careers," adds Gill. "Sometimes I am the optimistic one and Steve is cautious. Then the roles reverse. We keep each other going."

There is a further problem, however. Even if those gene therapy trials work perfectly, and patients' lung cells start to produce normal, healthy secretions, it will take a long time to detect. "We are not curing the condition," says Hyde. "We will merely be halting the gradual erosion of patients' lung functions. That will be a very successful clinical outcome, for it would add years and years to patients' lives, but it will take quite a while - certainly more than a year - to show that this is really happening. Then we may have to tweak things a bit and that could delay things further." Scientists' most confident estimate is that patients can expect effective treatments in three or four years.

Most will settle for that, though there is another crucial point that should be noted. If nothing else, the consortium project shows that turning genetic discoveries into medical treatments is an extraordinarily awkward business. When the results of the Human Genome Project were published in 2003 and revealed the structure of every single one of the 23,000 genes found in the human body, the work was heralded as a mammoth breakthrough. Soon we would be transforming these basic discoveries into new medicines, it was claimed. But the story of cystic fibrosis suggests something very different: that it will take decades, if not centuries, before scientists can turn that mass of knowledge into practical treatments. We should temper our expectations.

Nevertheless, consortium co-ordinator Eric Alton is optimistic. "When the Wright brothers built the Kitty Hawke, it flew only a few hundred yards. Yet within a couple of years, people were flying their planes for miles. Similarly we might only get a short distance with this first round of treatments but I am sure that very soon we will be making sustained progress for cystic fibrosis patients."

For many of them, that prospect - which has seemed so tantalisingly close for so long - is almost too much to contemplate. "We just want something to take the anxiety away, to know Anna will lead a reasonable life," says Katrina Dujardin. "The rest she can handle herself. She has learned to be a fighter."

• For more information, visit cftrust.org.uk

The genetics of cystic fibrosis

The genetic error that gives rise to cystic fibrosis is a widespread one. Among Caucasians, one in 25 people carry a faulty version of the CFTR gene. However, they display no symptoms and suffer no medical problems, for a simple scientific reason: our genes come in pairs that are inherited separately from our mothers and fathers. Possession of a normal CFTR gene compensates for a malfunctioning version. Such an inherited condition is said to be recessive. Thus most of Britain's two million carriers of cystic fibrosis genes are unaware of their status - unless they meet and settle down with another carrier. This occurs in approximately one in every 625 couples in Britain - around 90,000 sets of potential parents in which both man and woman are carriers of a CF gene. Even then, such couples are not destined to produce children with cystic fibrosis. Only in a quarter of pregnancies do both parents pass on their faulty version of the gene. When that occurs, their child has no healthy CFTR gene at all and will suffer from full-blown cystic fibrosis. In Britain, around 150 babies a year are born with the disease, including James Fraser Brown, the two-year-old son of Gordon Brown.

Intriguingly, the cystic fibrosis gene is much more prevalent among people of Caucasian origin than those with Asian or African ancestry. Among Asians it is carried by one in 90 people, and among Africans by one in every 65. This has led scientists to speculate that, in the past, people who are carriers may have enjoyed some protection against certain infectious diseases. Scientists have shown that carriers of genes for the inherited blood disorders thalassaemia and sickle-cell anaemia gain slight protection against malaria, for example. In cystic fibrosis, its association with the thickening of fluids in the body may therefore have given carriers slight protection against cholera and other diseases.