In 1989, Strachan reported 1 Strachan DP Hay fever, hygiene, and household size. BMJ. 1989; 299: 1259-1260 Crossref PubMed Scopus (3792) Google Scholar an inverse association between hay fever and the number and position of children in a household and in so doing launched the hygiene hypothesis. The initial rationale for this hypothesis was that later born siblings might have more infections acquired from their older siblings and other household members, and that this exposure might protect them from later development of allergic disease. This possible inverse association between infection or microbial exposure and allergic diseases opened new frontiers both for epidemiological and immunological research. That research had the additional explanatory attraction of both historical recency and western predominance, two features of the rising prevalence of allergic disease. Moreover, timely immunological mechanisms were at hand to support the hypothesis, whereby Th1 cytokines responding to infection would suppress Th2-mediated allergic responses. 2 Romagnani S Human Th1 and Th2 subsets: regulation of differentiation and role in protection and immunopathology. Int Arch Allergy Immunol. 1992; 98: 279-285 Crossref PubMed Scopus (334) Google Scholar Assessing the association of early life antibiotic prescription with asthma exacerbations, impaired antiviral immunity, and genetic variants in 17q21: a population-based birth cohort studyThe association between antibiotics and asthma might arise through a complex confounding by indication. Hidden factors that may increase the likelihood of both early life antibiotic prescription and later asthma are an increased susceptibility to viral infections consequent upon impaired antiviral immunity and genetic variants on 17q21. Full-Text PDF Open Access