The epidemic of asthma: too much allergen or not enough infection?

Asthma has generally been thought to result from exposure to allergens in infancy leading to atopy, and eventually to airway hyperresponsiveness. There is now evidence that implicates absence of childhood infections as a factor in development of asthma. Childhood infections seem to be important in normal maturation of the immune system, with asthma a manifestation of a persistent "immature" immune system.