One Sunday morning last November, I dropped my five-year-old son, Caspar, off at his grandma's house with his dad. They were going to spend the morning clearing out my husband's old bedroom. Caspar had gone along eagerly, hoping to unearth some of his dad's legendary old toys - Snoopy, or maybe even Roland Rat. Later that day Caspar was to go to a pirate party; we'd got out a scarf and earring, and were planning on some seriously twirly moustaches. "Shiver me timbers" was the morning's catch phrase. Little did we think that later in the day our vocabulary would expand to include the new catch phrases "anaphylactic shock" and "adrenalin EpiPen".

I took our younger son to visit a friend. An hour or so after I arrived, my husband called saying he was returning home with Caspar, who had started to cough badly. Caspar is asthmatic so we're used to coughs, but at home I found Caspar in an alarming state. Having left him only a couple of hours earlier, as bright as a button, I now found him lying semi-conscious on the sofa, coughing and clearly struggling to breathe. My husband had given him some Ventolin puffs, but they hadn't scratched the surface of the hideous rattle inside his chest. I tried to wake him, but he was angry and said he just wanted to sleep. He was covered in a rash that was spreading before my eyes. We needed to get him to hospital.

We jumped in the car, and I sat him on my lap and held his puffer over his mouth, continually pumping Ventolin and trying to keep him awake. All he could do was scratch; it was as if his whole body was erupting with rage. The journey to hospital seemed endless; we ran red lights, sounding the horn and keeping the hazard-warning lights on all the way. A nightmare - a scene from ER.

My husband had told me that Caspar had taken a chocolate from a glass dish at his grandma's. The moment he'd taken a bite, his body had rejected it - he'd tried to retch it up, he said it had made his tongue hurt. The sweet had contained hazelnuts. It was clear that Caspar was dangerously ill and it was the sweet that had caused this reaction. My mind went into overdrive. He's allergic to nuts... we didn't know. But there had been a couple of early warnings. He'd spat chocolate out before that he'd said had made his tongue hurt. Italian hazelnut chocolates. This was my fault.

We hit casualty at St Mary's in Paddington and were immediately directed by a knowing nurse to the paediatric resuscitation room. By this time Caspar's face had begun to swell hideously. We were suddenly surrounded by people. A young doctor took charge; he had eczema in his eyebrows and wore a Bugs Bunny tie and matching socks - it's weird the details one remembers. Caspar was carried on to the bed, an oxygen mask was placed over his mouth and his arm was jammed with an intravenous line pumping steroids and antihistamine into his system. Caspar screamed with the pain of the needle going in - no time for anaesthetic cream. His left lung, I was told, had shut down and he was going through an anaphylactic reaction - to something he had eaten.

We watched and waited in disbelief at what was going on around us. The doctors busied themselves, adjusting Caspar's mask, checking his pulse and listening to his chest. Slowly (actually, quite quickly, it just felt like for ever) the medication started to work. Caspar's breathing started to ease and the oxygen mask was removed. We were told he was going to be OK. A radiologist arrived with an x-ray machine; Caspar was terrified by the look of this machine but we reassured him that the man just wanted to take a picture of his lungs. Caspar took his skateboard T-shirt off and exposed his skinny little chest for the radiologist. From a crack in the door, I saw him pose and smile as the x-ray was taken.

Eventually we left the resuscitation room, exhausted, confused and a little hysterical. Caspar wanted to know if he was going to be late for the pirate party and what we were having for lunch. We were told we would have to stay in hospital overnight. Once on the ward we were visited by numerous doctors and nurses, all confirming that Caspar had had an anaphylactic reaction to the chocolate he had eaten. We'd got grandma to go through her bin and retrieve the half-eaten chocolate that had nearly killed my beautiful five-year-old boy - a Miniature Hero Picnic bar.

I was told that had I given Caspar a dose of Piriton - the over-the-counter antihistamine - immediately after he'd eaten the chocolate bar, he would probably have recovered within five or 10 minutes and we'd have avoided the whole hideous ER experience. Why didn't I know that? And why doesn't every other parent in the country know that?

Caspar would now need to carry a dose of adrenalin - an EpiPen - with him wherever he went. While he played with other sick children on the ward, I was taught, sitting on a rickety child's chair outside the play-house on the paediatric ward, how to use one. I practised stabbing the EpiPens into fat oranges, but quickly the nurse saw that my tears were hampering my aim and sweetly said I could come back in a few days and she'd show me again.

I'd always been a bit sceptical about food allergies, thinking nut allergies were a bit of an urban myth and writing off people with general food allergies as attention seekers - we'd once had a baby-sitter who was allergic to nearly all the contents of our kitchen except for the boys' Monster Munches and Kit Kats. And now the British Nutrition Foundation says that one in five people believes they have a food allergy, but the truth is that only 1-2% actually do. But how important this tiny percentage is and how seriously we must take real food allergy.

A year on, we've adjusted well to Caspar's condition - while life feels a little more fragile than before, our lives haven't really changed. Caspar has been tested for other foods and is allergic to all nuts except peanuts. I went through a phase of wanting to eliminate all nuts from the world, and I'm still unable to understand how supermarkets can display them in such a cavalier fashion. But Caspar is much more rational than me and enjoys telling adults to read the ingredients on the backs of packages before he eats anything. There are a few foods that he ate happily before that he can no longer go near - Cheerios "may contain traces of nuts", as may croissants from Starbucks or any other cafe where they sit too close to the almond tarts.

While everyone's concerned with Caspar's condition, it still shocks me how ignorant people can be - even clever people who love us. Weeks after Caspar's anaphylactic reaction, a relative arrived at our door with some chocolate money - which carried a clear warning on the packaging that the sweets may contain nuts. "Oh, can't he eat chocolate at all then?" "Best not to give him chocolates with nuts in, because they could kill him, which would be a real shame," I replied with a tight smile.