Allergy has been associated with relative affluence for over a century, and more recently a substantially lower prevalence of sensitisation to common aeroallergens has been demonstrated in parts of Eastern Europe compared with Western and Northern Europe. One of the strongest risk factors for atopy is small sibship size. This applies in both affluent and less affluent families, and in both Western and Eastern Europe. The protective effect of older siblings is stronger than that of younger siblings, suggesting that family structure in early life is important. A unifying explanation for all these observations would be that allergic sensitisation can be prevented by infections acquired during early childhood. Direct evidence in support of this hypothesis is still awaited, but immunological mechanisms have been suggested. A reduction in infection offers a more coherent explanation of past and current trends in allergy prevalence than does an increase in exposure to environmental chemicals.