Hygiene hypothesis

Article By:

Cohen, J. John Department of Immunology, University of Colorado Medical School, Aurora, Colorado.

Last reviewed:August 2019

DOI:https://doi.org/10.1036/1097-8542.331540

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• Hygiene hypothesis, published November 2009:Download PDF Get Adobe Acrobat Reader

A theory proposing that the absence of exposure to certain microorganisms during the postnatal maturation of a young child's immune system is detrimental to the child. The hygiene hypothesis (sometimes referred to as the "old friends hypothesis") postulates that humans have evolved with certain intestinal bacteria and worms whose presence can "train" the immune system not to overreact against harmless commensal organisms (often referred to as the microbiome, microbiota, and microbial flora) [Fig. 1]. As people have become more prosperous and are leading "healthier" lifestyles that are overly "clean," these organisms have been lost from the intestines, and the immune system is losing its natural balance. This predicts a significant increase in immune diseases, both allergic and autoimmune, which in fact has been observed in the industrialized world. See also: Bacteria; Human microbiota; Immunology; Intestine; Medical bacteriology; Microbiology; Microbiome; Public health

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