Yet the development of eczema has been linked to several drugs given routinely to children, including vaccinations, antibiotics and paracetamol.

This idea forms part of the 'hygiene hypothesis' which says that by 'protecting' children from exposure to dirt and germs, and by preventing disease from taking its full course in childhood, we are inadvertently destroying the immune system's ability to respond appropriately to infection and other stimuli.

During most of our evolution and from the moment of birth, our bodies are exposed daily to dirt and germs. It is thought that this exposure helped forge and maintain the immune system's sophisticated network of chemical pathways and specialised cells. According to the hygiene hypothesis, without the early input of dirt and germs, the immune system cannot respond appropriately when challenged.

The hypothesis came into being in 1989, when David Strachan, an epidemiologist at the London School of Hygiene and Tropical Medicine, noticed that children from large families where infections are likely to circulate freely were less likely to develop atopy (BMJ, 1989; 299: 1259-60).

His ideas were heresy to most doctors who believed that infections triggered allergies, not protected us from them. Some continue to resist this idea and argue that routine jabs have 'saved' us from killer diseases such as polio and tetanus. But, the other side of the argument is that they also provide the wrong sort of stimulation to the immune system. Instead of strengthening it, they overstimulate one part at the expense of another, leaving us even more vulnerable to allergies and other immune problems.

In the intervening years, the hypothesis has been more fully developed. While the common media interpretation of the hygiene hypothesis is that childhood diseases train the immune system to respond vigourously to stimuli, Graham Rook, an immunologist at University College London, says this is rather too simplistic. He also points out that the name 'hygiene hypothesis' is misleading since hygiene per se is not a bad thing and, indeed, better hygiene is responsible for many gains in human health. It is obsessive measures toward cleanliness, killing germs and suppressing illness that are problematical.

In the hygiene hypothesis, atopic diseases form part of yet another triad that includes immune diseases such as MS and insulindependent diabetes, and inflammatory bowel diseases such as ulcerative colitis and IBS.

"In each case," says Rook, "it is not that the immune system isn't working, but that it is working too well. Often it is overresponding to minimal amounts of stimulus and to things which should never elicit an immune response. What's missing is the mechanism which shuts the immune reaction off."

The data on vaccination and atopy are remarkably consistent. There are studies showing that the more exposure to infections such as tuberculosis (Science, 1997; 275: 77-9), measles (Lancet, 1996; 347: 1792-6) and hepatitis A (BMJ, 1997; 314: 999-1003), the less the atopy.

In one study of children who received the DTP (diphtheria, tetanus and pertussis) and polio vaccines, 23.1 per cent had asthma episodes, 22.5 per cent had asthma consultations, and 30 per cent had consultations for other allergic illnesses (Epidemiology, 1997; 8: 687-80).

In a more recent study from the UCLA School of Public Health, the DTP vaccination was again shown to increase the incidence of allergies. The risk of having a history of asthma was twice as great among those who received the vaccine, with the highest risk in children who were 5-10 years of age.