Yale researchers have found new supporting evidence to the so-called "hygiene hypothesis" - the theory that a lack of exposure to parasites, bacteria and viruses in the developed world may lead to increased risk of diseases like allergies, asthma, and other disorders of the immune system.

In an animal study published in Nature, they found that exposure to certain non-pathogenic gut microorganisms helped prevent the onset of Type I diabetes in non-obese diabetic mice. From the report:

Non-obese diabetic (NOD) mice develop the disease at different rates after natural breeding, depending upon the environment where they are kept. Previous research has shown that NOD mice exposed to killed (i.e., non-active) strains of tuberculosis or other disease-causing bacteria are protected against the development of Type I diabetes. This suggests that the rapid "innate" immune response that normally protects us from infections can influence the onset of Type 1 diabetes.

In the Nature paper, teams led by Li Wen at Yale and Alexander V. Chervonsky at the University of Chicago showed that NOD mice deficient in innate immunity were protected from diabetes in normal conditions. However, if they were raised in a germ-free environment, lacking "friendly" gut bacteria, the mice developed severe diabetes. NOD mice exposed to harmless bacteria normally found in the human intestine were significantly less likely to develop diabetes, they reported.