Asthma is occurring in epidemic proportions with more than 300 million affected subjects worldwide. In almost all cases the disease has its onset in early childhood, with 80–90% of all cases initially being diagnosed before the age of 6 years.^1,2

It was not always so. In the early 1970s the prevalences of asthma and allergy were roughly half of what they are today, and although the onset of the asthma epidemic started insidiously and cannot be precisely documented, it has several interesting and important features that have defied a unified explanation until now. There is clearly a North/South equatorial gradient with Western industrialised countries furthest away from the equator (New Zealand, Australia, the UK) having the highest prevalence worldwide. There is also a clear urban/rural gradient among poorer Third World countries, and a First (industrialised) World/Third World gradient with the lowest asthma prevalence occurring in rural areas in Third World societies. A very important feature of the epidemic is that it is not only asthma that has increased. A host of other autoimmune diseases—such as multiple sclerosis, type 1 diabetes and Crohn’s disease—have all increased dramatically as well.³ So it is not just Th2 diseases like asthma, allergic rhinitis, eczema and food allergy that have increased, but also Th1 autoimmune diseases. Less directly connected to the epidemic is the seasonal gradient in incidence with a peak in the late winter/early spring. Finally, in the USA the asthma epidemic seems worse among the urban poor, particularly among minorities (African-Americans and Puerto Ricans).

One of the theories to explain these disparate epidemiological findings is the hygiene hypothesis, which posits that the decrease in early childhood infections leads to “missing immune deviation” …