Hygiene theory and allergy and asthma prevention.

Epidemiological trends of allergic diseases and asthma in children reveal a global rise in their prevalence over the past 50 years. Their rapid rise, especially in metropolitan locales, suggests that recent changes in modern environments and/or life styles underlie these trends. One environmental/life style factor that may be contributing to this trend is called the hygiene hypothesis: that naturally occurring microbial exposures in early life may have prompted early immune maturation and prevented allergic diseases and asthma from developing. Subsequently, children raised in modern metropolitan life styles, relatively devoid of this natural microbial burden, may have under-stimulated immune systems in infancy, thereby allowing for the 'allergic march'- a pattern of pro-allergic immune development and disorders that occurs in early life. Over the past 15 years, hygiene theorising has evolved in shape and substance, in part due to a growing and strengthening burden of evidence from epidemiological, translational and basic research. What was speculation may be key clues to allergy and asthma prevention. The objectives of this article are to summarise the epidemiological trends and allergic march of childhood that may be explained by hygiene theory, to overview current hygiene theory paradigms and to exemplify the strengthening epidemiological evidence in support of the hygiene theory, using bacterial endotoxin exposure as a prototypical example.