Sterile environment can cause the development of allergies. 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. It is hypothesized that the TH1 polarized response is not induced early in life leaving the body more susceptible 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.
Although the idea that exposure to certain infections may decrease the risk of an allergy is not new, David P. Strachan was one of the first people to formally suggest the theory in an article published in the British Medical Journal (now the BMJ), in 1989. In this article, the hygiene hypothesis was proposed 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. It is used to explain the increase in allergic diseases that has been seen since industrialization, and the higher incidence of allergic diseases in more developed countries. The hygiene hypothesis has now expanded to include exposure to symbiotic bacteria and parasites as important modulators of immune system development, along with infectious agents”