Hygiene Hypothesis

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 T_h1 polarized response early in life so as we grow up we are more prone to developing T_h2 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.