In recent decades there have been marked increases in asthma prevalence in Western countries. More recently, asthma prevalence has peaked, or even begun to decline, in Western countries, but many low and middle income countries are now beginning to experience increases in prevalence (although there is no evidence of increases in prevalence in India to date). “Established” risk factors for asthma cannot account for the global prevalence increases, or the international patterns that have been observed, or the recent declines in prevalence in some Western countries. It seems that as a result of the “package” of changes in the intrauterine and infant environment that are occurring with “Westernization”, we are seeing an increased susceptibility to the development of asthma and/or allergy. There are a number of elements of this “package” including changes in maternal diet, increased fetal growth, smaller family size, reduced infant infections and increased use of antibiotics and paracetamol, and immunization, all of which have been (inconsistently) associated with an increased risk of childhood asthma, but none of which can alone explain the increases in prevalence. It is likely that the “package” is more than the sum of its parts, and that these social and environmental changes are all pushing the infants’ immune systems towards an increased risk of asthma.