At four months of age, Leo started producing oil out of his bottom - it flowed out like pure sunflower oil when I changed his nappy. At first I thought I'd been eating too many curries. "Has anyone else witnessed this?" asked the health visitor. They hadn't. I was alone with the image of it - and the worry.

When Leo was born he weighed 9lb 9oz. "A real bruiser," announced the midwife proudly. "What is she so proud of," I remember thinking. "She hasn't just gone through 22 hours labour to have him sucked out with a sterilised Dyson attachment." I spent the next few days smugly telling everyone that Leo would soon outgrow his carrycot.

At three months, Leo weighed 10lb 3oz and had plenty of room in his carrycot. Other babies were getting bigger, but Leo seemed to be shrinking and my heart was breaking.

All his symptoms were put down to a bout of brochialitis at the age of five weeks. This meant an eye-opening stay in hospital; breastfeeding two hourly with no meals provided. The consultant was convinced Leo would "sort himself out", and said he had "seen a lot worse". I lay awake at night, raking up my limited knowledge of biology to try and work out what could be wrong - worms? In my heart of hearts I knew something wasn't right. I reported the oily nappies to the GP, who said: "Classic symptoms of cystic fibrosis ... but don't worry, I'm sure it's nothing serious." Yes, I did go home and worry my socks off and yes I did go home and grab every book with any fleeting reference to CF. That was a mistake: "... but now CF sufferers can live up to the age of 30 ..."; "... are generally unhealthy ..."; "... all males are sterile ..."

I cried on the phone to various people, who all reassured me he must be OK - no babies had slipped through the Guthrie heel prick test (done at birth) for years. I relayed my horror to the consultant who called Leo "fatty" and offered to arrange a "sweat test", which is the diagnostic tool for CF. The technician cheerily informed me that they assume the results will be normal; any problems and the whole world is phoned. I walked away and knew they would phone.

Finally, Leo is getting the treatment he needs and is starting to outgrow his carrycot. Everyone is being very positive, the medical team are the best thing since sliced bread; but every now and again my heart breaks.

From a factual point of view, CF is an inherited disease which results in too much mucus being produced in certain organs in the body, mainly the lungs and the digestive system. Hence Leo's constant cough and his "oil-rig" bottom - the mucus was stopping him from absorbing fat. The oil became apparent at four months when I introduced bottle feeding; apparently the fat in powdered milk is harder to digest than in breast milk.

CF is the UK's most common life-threatening disease, affecting more than 7,000 people. Genetically, both parents must be carrying the defective gene that causes CF, but even then you must be unlucky: you then have a one in four chance of your child being born with the disease.

I sometimes wonder: "What if we had had sex the night before or the next night?" The sexual lottery might not have thrown up CF ... But I've also come to realise that Leo's genetic combination also contains ... Leo. His placid, patient, smiley warmth.

We have seen the funny side: "life begins at 40", "he won't have to worry about a pension". We have to be positive for his sake; in 10 years' time, gene therapy may mean a more normal life expectancy - Leo can then expect what most of us take for granted.

• For more information, call the Cystic Fibrosis Trust on 020-8464 7211.