The news last week of the use of gene therapy in making a near-blind young man see again must have touched everyone. What a magnificent achievement it was.

It wasn't just a story about preventing blindness - that would have been huge enough. It was also proof that gene therapy really does work, and that we are in the infancy of a whole new range of treatments that will eventually cure diseases for which we have no effective treatment today.

The eye treatment involves two principles. The first is that it corrects an inherited gene defect. In children and teenagers with inherited eye disease, the defect causes the retinal' cells in the screen' at the back of the eye to die off. The result is blindness in early adulthood.

What the researchers did was to identify the faulty gene, find the correct one, then work out a way to insert the good' gene into the cells. Once the new gene is in place, the affected cells will then survive and even recover a lot of their lost abilities.

Imagine the amount of research that went into the detection of the gene defect and the isolation of its replacement. Then think of how to get the new gene into the right layer of cells, and then how it could be spread throughout it, to cover the whole of the back of the eye.

It isn't just a matter of injecting the gene material into the tissues - that wouldn't help at all. It would just lie outside the cells, making things worse.

So the second principle was getting the gene to where it is needed. The researchers needed a way of carrying it inside the dying retinal cells, so that they could use it to recover. There is only one natural way of spreading material throughout a tissue. We feel it every time we fall foul of an infection.

Viruses use substances on their outer surfaces to penetrate through cells and to spread between them. If you have had a common cold or a viral gastro-enteritis you will understand exactly what viruses can do. They spread throughout your nose and throat, or your gut, like wildfire.

The researchers reasoned that if they could find a virus that would infect' the retina, attach the new genetic material to the virus, and inject the virus-gene combination into place, the combination would spread the material through the cells for them.

Of course, they had to find a way of disabling' the virus's ability to cause illness before they could do so. Imagine the work that went into finding a virus that could spread without damaging tissues and that could carry the gene with it.

The results have been fantastic. It's true that so far we have heard about just one patient - a teenage boy who can now negotiate a maze in very dim light in a fraction of the time that he took before the treatment. What a brilliant lad he was.

The really encouraging part of his story was that his sight is still improving, months after the injections, proving that the new genes are still working. It wasn't just a flash in the pan: from facing a future in total darkness he will be able to see enough to live an independent life.

The researchers are now going to concentrate on younger children with a variety of genetic diseases. They have already identified the genes they need to replace, and the future seems exceptionally bright.

The spin-off from this research is massive. Experts in infectious diseases are working in tandem with geneticists to identify other virus-gene combinations as possible cures for inherited diseases of other systems - the bowel, the lungs, even the brain and nervous system are all candidates.

We know a lot about viruses that spread among them, and how to disable them, and our knowledge of genetics, and how genes can go wrong, has mushroomed beyond our wildest expectations.

We are already using this knowledge to help children with cystic fibrosis - using viruses that would normally cause colds and bronchitis to place new genes into their lung tissues.

Imagine a time when we can carry' with designer viruses' medicines into the target spots for people with diseases like multiple sclerosis, Parkinson's disease and Alzheimer's. Or when we can programme viruses to enter only cancer cells, and attach to them medicines that will kill them, and them alone.

None of this is fanciful - we are already well on the way with the cancer-killing systems. If there ever was a time for optimism and belief in modern medicines, this is it.