Over the past decades, the increasing rates of allergic conditions among affluent societies have posed a heavy burden on healthcare systems. Cross-sectional studies such as the International Study of Asthma and Allergies in Childhood (ISAAC) have confirmed that atopic diseases such as atopic dermatitis, asthma, and seasonal allergic rhinoconjunctivitis represent major health problems in many countries, particularly in childhood.
During the past 2 decades, two general hypotheses have been proposed in the literature in connection with the observed increases of atopy and asthma in childhood:
- New risk factors that were not known several decades ago might have become relevant in connection with nutrition, environmental exposure, and lifestyle. Protective factors related to a more traditional lifestyles common in the past might have been lost, which could have led to increased susceptibility to atopic diseases.