Hygiene Hypothesis...

The hygiene hypothesis states that a lack of early childhood exposure to infectious agents increases susceptibility to allergic diseases.

The hygiene hypothesis was developed to explain the observation that hay fever and eczema, both allergic diseases, were less common in children from larger families, which were presumably exposed to more infectious agents through their siblings, than in children from families with only one child.

The hygiene hypothesis has been extensively investigated by immunologists and epidemiologists and has become an important theoretical framework for the study of allergic disorders. Despite this, the infectious microorganisms or infectious microorganism-components believed to be responsible for these effects have yet to be identified and incorporated into medical practice and the contribution of hygiene levels to the rise of allergic disease has yet to be established.