The next challenge is to mimic safely this protection against allergy and asthma

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Although infectious diseases are by no means defeated, the past 100 years have seen a dramatic decline in some previously common childhood infections. Many serious viral and bacterial infections can now be prevented or treated by vaccination or antibiotics. In contrast, the prevalence of asthma and atopic disease has increased, particularly during the past 30 years. This increase is certainly not accounted for by a change in genetic risk factors: genetically similar populations in East and West Germany had very different rates of asthma before unification (although former east Germany is now catching up with the west1). In a landmark study of hayfever, hygiene, and household size in 1989 Strachan proposed that improved hygiene was the factor that explained this rise.2

The immunological arguments that underlie this “hygiene” hypothesis can be …