Any 39-year-old woman will have her own reproductive history to tell. If she has children, there is the tale of her body's nine months in pregnancy, the first stirrings of the child inside, the sweet kicks and the "I can't do it" mumbled before the job is done. There may be the spectres, too, of any children lost to miscarriages or terminations. All notches carved into the timber of the female self.

But I have an extra burden. My two-year-old son David has a life-threatening disease and it is my fault: I gave it to him. He has Chronic Granulomatous Disorder (CGD), a very rare primary immune deficiency that affects about just one in a million people worldwide. He has a faulty bone-marrow gene, which means his white blood cells are unable to produce the bleach-like chemicals needed to kill non-oxygen-producing bacteria and fungi, leaving him wide open to life-threatening infections.

Generally, CGD is an inherited disease, the missing chromosome X-linked. I am the carrier and have passed it on to my two beautiful children. My four-year old daughter Esther should be all right. Her extra X chromosome will mean that her immune system should function normally. But in David's case it is, if left untreated, fatal. Blood tests have confirmed that my mother and sisters are not carriers, suggesting that some spontaneous mutation arose in my case, yet I feel guilty and am full of self-hatred.

Had I known that I was a carrier, I am certain I would have chosen not to have children. Nor would I have willingly chosen to see a child of mine suffer, or face the possibility of my son dying while I am alive.

The options facing confirmed carriers today are brutal: prenatal tests on womb tissue (chorionic villus sampling) or fluid (amniocentesis) can show whether the baby will have CGD. If the child is male and has CGD, therapeutic termination is available.

Esther is a carrier too. If she has children with an unaffected man, there is a 50% chance that any sons born would develop the condition. Though her daughters would not have CGD, there is a 50% chance they would be carriers. When I ask the consultant at Great Ormond Street hospital about Esther's future, he says that in 30 years' time people will not make love to have children.

He is referring to the use of the technique called Pre-Implantation Gender Determination (PIGD), which involves using IVF (invitro-fertilisation) treatment and enables couples to choose the sex of their baby. This complicated technique is available today and, in years to come, its highly developed form could perhaps offer a comprehensive genetic screening of the foetus-to-be.

The consultant speaks as a physician who is keen to see the eradication of disease. But to me, still reeling from the pain of a new diagnosis, the statement reads like a rejection of David - an indictment of our son, who was conceived in love, and bought forth into the world in good faith.

David - it means "beloved one" in Hebrew - has always been a very happy, active boy. And very independent, too: we found him once halfway up a hill and on his way to our B&B at a party in Shropshire. He was also very big for his age, making diagnosis more difficult in his case, as children with CGD are typically on the smaller side.

Eight months ago, six boils appeared on David's nose, for which the GP prescribed anti-bacterial Fucidin cream. Then came the high fevers and awful night sweats. David was finally admitted to the paediatric liver unit at King's College hospital, after the ultrasound of the liver taken at the local hospital that day showed "masses". He had been fighting three liver abscesses all the while, and the following week, a simple test called the NBT (nitro blue tetrazolium) confirmed that David had Chronic Granulomatous Disorder. Our hospital stay lasted 11 weeks.

I was acquainted with illness, but disease has a different feel to it; you just sense it. David had had 13 hospital admissions by the time he was two and a half, and despite being given the all-clear by a distinguished immunologist, an uneasy feeling remained.

What does the future hold ? At the moment, while we await a resolution to the liver abscesses, David is on a three-month cocktail of oral antibiotics, with blood tests taken weekly to check a fall in his blood inflammation markers. Once we are over this episode, David will start a lifetime course of daily prophylactic antibiotics and antifungals. Any fever above 38C will need prompt investigating by the local paediatric ward.

This can happen at any time; I started this article at the local hospital, where David was admitted with a fever and a cough. Intravenous antibiotics were immediately started through David's Hickman line (a plastic tube inserted into and then out of the chest through an incision at the neck) and blood taken. Luckily, on this occasion the combined results of a chest x-ray, ultrasound and blood cultures showed up nothing more sinister than a viral infection, which David's body can fight. Inflammation invariably causes problems too for people with CGD - David has colitis, an inflammation of his large bowel which regularly causes blood-stained diarrhoea.

I cannot imagine how such abrupt changes of gear - the very physical medical intrusion that goes with disease, or the swift disappearance of home - can ever be accommodated psychologically: my daughter is suffering, too.

And despite our wonderful consultant's admonitions to give the boy a normal life, the list of things not to do is prohibitive. David cannot paddle in streams or splash about in puddles. He cannot dig in the garden, go near cut grass or jump in piles of leaves. Caves, barns, sheds and lofts are off-limits, as are any buildings where works are taking place.

Cure exists in the form of bone marrow transplantation, which depends on finding a compatible donor - with siblings sometimes presenting as exact matches - but carries very serious risks. Gene therapy is in its infancy and is at present only offered as part of clinical trials.

In my daydreams, David is cured by gene therapy and we set off in a camper van to see the world, or just garden like we used to. In the meantime, black is a colour I will not wear, cannot bear looking at. I start to dress our children in the bright colours and patterns of the children of the mountains of the Iran of my childhood.

· For more information about CGD, see cgd.org.uk