The germless theory of allergic disease: revisiting the hygiene hypothesis.

Rising rates of allergic disease accompany the healthier benefits of a contemporary westernized lifestyle, such as low infant mortality. It is likely that these twinned phenomena are causally related. The hygiene hypothesis states that allergy and increased longevity are both consequences of reducing infectious stressors during early childhood for millennia. Mechanistic explanations for the hygiene hypothesis have typically invoked the T-helper-type 1/2 (T(H)1/T(H)2) model. Here, we discuss why we favour a broader 'counter-regulatory' model--one that might also explain the increasing incidence of autoimmune disease in westernized countries.