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The findings support previous research that suggests a lack of exposure to germs during early childhood changes how the immune system reacts to bacterial threats—the “hygiene hypothesis” which emerged in the late 1980s to explain why allergies like hay fever and eczema were less common in children from large families, who were presumably exposed to more infectious agents. In one such study, germ-free mice died after being fed doses of the pathogenic bacterium Listeria monocytogenes containing as few as 100 cells. On the other hand, normal mice survived up to 1 billion cells of the same germ.

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