The “Hygiene Hypothesis” and the Lessons Learnt From Farm Studies
Erika von Mutius
Abstract
We celebrate the 30th anniversary of the "hygiene hypothesis", which has been a cornerstone for research into asthma and allergic diseases for many of us. It appeared while we witnessed the rapid increase of these conditions in the westernized world by the end of last century (1). It has stimulated thought of many researchers resulting in numerous more or less modified hypotheses meandering in diverse gestalt through the scientific landscape. It all started with an epidemiological observation about significantly decreased risk of allergic sensitization and hay fever in subjects having many siblings. This observation was counterintuitive at that time when the prevalent paradigm stated that viral infections cause asthma. However, the observation was confirmed many times in independent populations and is one of the most robust epidemiological findings in the context of allergy (2). Over the years the epidemiological gestalt changed from siblings to day care, oro-fecal and other infections and then to farm exposures. Interestingly, the farm effect is independent of the "sibling effect" (3). The gestalt also took on various immunological garments from a Th1-Th2 dichotomy to regulatory networks. Lately, the technological progress allowing exploration of the world of microbiomes has revitalized the debates around the "hygiene hypothesis" with tantalizing findings from mouse experiments and population-based studies.