In medicine, the hygiene hypothesis states that a lack of early childhood exposure to infectious agents, symbiotic microorganisms (e.g., gut flora or probiotics), and parasites increases susceptibility to allergic diseases by suppressing natural development of the immune system. Because of this we fail to induce a Th1 polarized response early in life so as we grow up we are more prone to developing Th2 induced disease. The rise of autoimmune diseases and acute lymphoblastic leukemia in young people in the developed world has also been linked to the hygiene hypothesis.
There is some evidence that autism may be caused by an immune disease; One publication speculated that the lack of early childhood exposure could be a cause of autism.
Read more about Hygiene Hypothesis: History, Mechanism of Action, Breadth of The Hypothesis, Supporting Evidence, Helminthic Therapy, Alternative Hypotheses
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“It is an hypothesis that the sun will rise tomorrow: and this means that we do not know whether it will rise.”
—Ludwig Wittgenstein (1889–1951)