THE hygiene hypothesis posits that certain diseases-notably asthma, eczema and type-1 diabetes-which are becoming more common than they once were, are caused in part by modern environments being too clean. The diseases in question result from misfunctions of the immune system. The hygiene hypothesis suggests such misfunctions are the result of children's immune systems being unable to learn, by appropriate exposure to viruses, bacteria, fungi and parasitic worms, how to respond properly.
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