Until recently, most studies reported that asthma prevalence has increased in recent decades. The best indication of what is now happening globally will be provided by Phase III of the International Study of Asthma and Allergies in Childhood (ISAAC) study. Some individual ISAAC centres in Western countries, as well as several studies in adults, have already reported no increase or even a decrease in asthma prevalence over the last 10 years. 'Established' risk factors for asthma cannot account for the global prevalence increases, the international patterns or the recent declines in prevalence in some Western countries. It seems that the 'package' of changes in the intrauterine and infant environment occurring with 'Westernisation' is causing increased susceptibility to the development of asthma and/or allergy. SUMMARY The 'package' includes changes in maternal diet, increased foetal growth, smaller family size, reduced infant infections, increased use of antibiotics and paracetamol and immunisation, 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 our immune systems in the same direction. To know what that direction is requires that better aetiological theories of asthma are developed to replace the allergen theory, or to incorporate it as a special case. Global comparisons of asthma prevalence and assessment of time trends will continue to play a major role in this process.
