There has been a surprising outbreak of the use of the c-word among medical researchers over the past few days. Normally cautious in their language, they have nevertheless been wielding the term “cure” when discussing the long-term potential of two separate treatments for inherited ailments that were announced last week. Such enthusiasm is striking.

In one case, scientists based at St Bartholomew’s, London – who have been working on the inherited bleeding disorder haemophilia A – outlined how they had used a virus to carry the gene for the blood-clotting chemical, factor VIII (which patients lack) to their livers. Production of the missing chemical was restored and their bleeding halted. The development, according to the World Federation of Hemophilia, now points “the way to a cure” for the condition, which affects around 400,000 people worldwide.

In the other case, scientists led by Professor Sarah Tabrizi, of University College London, revealed they had found a way to suppress the build-up of harmful proteins in patients suffering from the incurable degenerative condition, Huntington’s disease. Injections of the drug Ionis-HTTRx destroyed genetic messengers that directed the manufacture of these proteins. Dementia experts hailed the news as a “tremendous step forward” because it could be used not only to target proteins involved in Huntington’s but in other neural conditions, such as Alzheimer’s disease.

Please note: neither the work on haemophilia or the work on Huntington’s can yet be termed as cures, but they point to the prospect of effective treatments being developed in future. Hence the outbreak of the use of the c-word last week. It is also worth noting that both techniques are forms of gene therapy, in which a mutated gene is either replaced with a healthy copy (as with the haemophilia trials) or is inactivated or “silenced” (as with the Huntington’s work). And that development is also worthy of note.

Twenty-five years ago, as molecular biologists first honed the tools that now allow them to manipulate DNA at will, it was claimed that gene therapy could soon free humanity from the misery of countless conditions including haemophilia, Alzheimer’s and some cancers – simply by altering a person’s genetic make-up. It proved to be an overly ambitious goal, as gene therapy pioneer Professor Eric Alton, of Imperial College London, acknowledged. “Over the past couple of decades, the reputation of gene therapy has gone from being a cure for all known diseases to something that you wouldn’t give your dog.”

Part of the problem lay with the deaths of some patients during trials of different gene therapies. However, the main reason for gene therapy’s fall from grace was its simple failure to produce the goods as quickly as predicted. Fiddling with our genes proved to be a lot trickier than anticipated by some scientists.

This point is acknowledged by Professor James Gusella, the Harvard University geneticist who first pinpointed the gene that causes Huntington’s and who was always cautious about the likely rate of developing gene therapy. “You have to appreciate that the symptoms of the disease, like any other illness, are the end result of a long series of processes that take place inside the body,” he said.

And it takes time to understand that pathway. It begins with the cause of a particular disease – an infection or the inheritance of a gene – and then leads through a series of knock-on effects that eventually produce symptoms. Researchers then have to pinpoint which stage is the one most susceptible to intervention. It has taken 25 years to get to this position with Huntington’s disease.

The reputation of gene therapy went from being a cure for all known diseases to something you wouldn't give your dog Professor Eric Alton

Alton agreed progress has been slow. For decades, his team has been developing gene therapy treatments for cystic fibrosis – an inherited lung and digestion disorder that affects 10,000 people in Britain – and only now is it emerging as a potential treatment. “However, the crucial point is that we getting there.”

And that point needs stressing. Others may have dismissed the prospects of gene therapy, after its initial hyping, but its advocates still ploughed on, bouncing back after each setback, until success was eventually achieved. Apart from last week’s developments, gene therapy has also helped treat immune conditions and some forms of blindness. In each case, it has taken a great deal of hard graft to reach these goals.

This is the way that science progresses, of course – not along an unswerving trajectory towards the truth but by staggering through disappointments, reversals and reappraisals. “Progress is usually a very slow, drawn-out business that features many setbacks and occasional small advances,” says Professor Robert Lechler, of King’s College London. Eureka moments of triumphant discovery are certainly the exception.

The fact that science is rarely presented this way has much to do with the natural enthusiasm of scientists for their particular projects and with the tendency of journalists like myself to push them into guessing when their work might be ready for clinical use. Five years is the usual answer I get, no matter what the trials involve.

In the case of gene therapy, the answer should have been about 25 years – though I would have had difficulty selling such a remote prospect as a story to my newsdesk, while the poor scientists would have found it hard to acknowledge how long they still had to toil to achieve their goal. Hence those over-optimistic claims.

That last issue should be kept in mind when considering other medical developments that are being worked on, added Lechler. “Both stem cell science and gene editing have been highlighted as having enormous potential to treat illnesses – and of course they do. However, it may take longer than we expect – certainly if the example of gene therapy tells us anything. We shouldn’t disparage these technologies if they don’t reach fruition straightaway, however.”

Scientists are human and are sometimes prone to exaggeration and a bit of self-deception for good measure. But they do seek the truth. Even in relatively sane periods, that urge should be cherished. In today’s truth-tarnished times – assailed as we are by the egregious advocates of unreason who promote climate change denial, vaccine repudiation and other deceptions – we have never needed an example of dedicated, effective science so desperately. Researchers – working in the face of setbacks for decades – have now come close to finding treatments for some truly terrible conditions and have rightly been hailed for their work.

Their achievements should also be borne in mind the next time someone challenges a scientific truth just because it is inconvenient. That truth, no doubt gained at some personal cost, is something to be treasured, not denied.