The main reason for the asthma epidemic in industrialized countries is unknown. While childhood mortality from acute respiratory infection is still high in developing countries where asthma prevalence is low, there might be a suppressed natural selection in industrialized countries with a high asthma prevalence. Children with an inborn Th1 maturation defect might survive by better health care and antibiotic use at the cost of higher asthma and allergy rates. Another distinct group of children maybe represented by those having an environmental Th1 trigger deficit where the immune systems is not being sufficiently exposed in early life. Both, a Th1 maturation and a Th1 trigger defect may contribute to a dual Th1 allergy theory.