The prevalence of asthma varies widely throughout the world. We now believe that asthma is due to airway inflammation caused by an imbalance of the T-lymphocytes in the airway. The atopy that drives asthma is due to a predominance of Th2 lymphocytes in the airway. This paper links the prevalence of asthma inversely to the prevalence of tuberculosis and enteric infection. We will argue that the reason we observe such marked variations in the prevalence of asthma around the world is the fact that in the developing world there is a survival advantage in the fetal immune response to mature from Th2 to Th1 lymphocyte predominant. A Th1 response is required to combat infectious diseases such as typhoid and tuberculosis. Data from the World Health Organization confirms that these two infectious diseases occur very rarely in those communities where the asthma is high. It may be that the clean and infection-free environment of Australia and New Zealand is responsible for the region of Oceania having the dubious honor of having the highest prevalence of asthma in the world. (C) 2000 Harcourt Publishers Ltd.