Old friends for breakfast

We expect patterns of disease to change in parallel with economic development, but the existence of simultaneous increases in diverse chronic inflammatory disorders came as a surprise. Developed countries have more allergic disorders, more autoimmune disease (for example Type 1 diabetes and multiple sclerosis) [1] and more inflammatory bowel disease (IBD; ulcerative colitis and Crohn’s disease) [2]. Influenced by the increasing incidence of Th2-mediated allergies, but largely unaware of the simultaneous increases in the Th1-mediated disorders, workers in the field of allergy developed the concept that became known as the Hygiene Hypothesis. This hypothesis, though essentially correct, rapidly spawned three untenable interpretations which delayed its acceptance. However these have now been shown to be unlikely, and a new interpretation, the “Old Friends” mechanism, is looking much more promising.