Abstract / Summary
Over the past half century, allergic diseases such as asthma, atopic eczema, hay fever and food allergy have risen dramatically, particularly in industrialized, urbanized societies. In 1989, epidemiologist David Strachan proposed what became known as the 'hygiene hypothesis': a lack of early-childhood exposure to infectious agents, and more broadly to a diverse microbial environment, may disturb the development of the immune system and predispose a child to allergic disease. Since then, the hypothesis has evolved considerably. It is no longer understood as a simple call to abandon cleanliness, but rather as a recognition that the human immune system is calibrated by lifelong contact with beneficial microorganisms, the so-called 'old friends', including the gut microbiota.
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