Allergic disorders are the chronic diseases of greatest pediatric morbidity, affecting over 25 % of the pediatric population. Indeed, this situation has been referred to as an "allergic epidemic". In comparison with asthma, atopic dermatitis and allergic rhinitis have been less extensively investigated, although this does not mean that they should be regarded as minor disorders but rather as alterations that affect the quality of life of the patients and their families, which generate considerable direct and indirect costs. Despite an important research effort, the reason for this allergic epidemic is not well known. These are multifactor disorders without a single causal agent, in which the most important component is the genetic predisposition of the patient (atopy), modulated by environmental factors, exposure to allergens, infections and irritants, among others. A confounding element is the fact that the concept of allergic diseases encompasses phenotypes of rhinitis, atopic dermatitis or asthma in which no IgE-mediated atopic mechanism is demonstrated, and which can manifest in a way similar to true allergic phenotypes. Differentiation between the two is difficult to establish on the basis of self-administered questionnaires alone, in the absence of a precise etiological diagnosis. The present article reviews the numerous factors suggested to be responsible for the increase in allergic diseases recorded in the last few decades, and for the differences in prevalence observed among centres. For most of these factors the results published in the literature are contradictory, in some cases due to a lack of control of the associated interacting or confounding factors. Consensus exists for only some of these causal factors, such as the established parallelism between the increase in allergic diseases and the reduction in infectious processes on one hand, and the increase in particles generated by diesel fuel combustion on the other. In addition, the implicated factors could act differently (and in some cases even antagonically) upon atopy and on the different disease phenotypes, thereby complicating the study of these interactions even further.

In the last decades there has been an increase in allergic disease throughout the world, particularly in children. Attempts have been made to identify the causes of this "allergy epidemic" in environmental changes and changes in population hygiene, lifestyle, socioeconomic level, and eating habits that would exert epigenetic effects. Dietetic hypotheses have been mainly focussed in long-chain polyunsaturated fatty acids, vitamin D, antioxidants, Mediterranean diet, and fruits, vegetables and fish consumption. Although the data suggest a certain association between diet and the development of asthma/allergy, there is no evidence that diet has an impact upon the prevalence of such diseases after early infancy. If indeed there is such an impact, it is likely to be confined to the prenatal period and the first months of life - when it is still possible to modulate the development of the respiratory, digestive and immune systems. Thus, once the most appropriate preventive measures have been defined, these should be implemented during pregnancy and lactation. The existing scientific evidence is unable to recommend any primary preventive measure in the general population or in different population subgroups. Special or restrictive diets in pregnant or nursing women are not indicated. Exclusive breastfeeding for six months is questioned, since solid foods should begin to be introduced at around four months of age. Once the atopic process has started, no nutritional strategies have been found to be effective as secondary or tertiary preventive measures. Longitudinal studies in cohorts of pregnant women or newborn infants could help clarify these issues.