Infectious diseases, including tuberculosis, malaria, hepatitis, pneumonia, dysentery, and helminth infestations, still constitute a profound threat in developing countries. Curiously, their decline in high-income societies is paralleled by an unprecedented emergence of allergic disorders, notably asthma and atopy, and chronic inflammatory and autoimmune diseases, such as Crohn's disease, type 1 diabetes, and multiple sclerosis. Several changes in lifestyle are associated with this transition, including diminished exposure to soil and animals, nutritional bias, obesity and increased exposure to pollutants and antibiotics, which all impact the intestinal microbiota. Understanding the mechanistic links behind the epidemiological observations, the complexity of a changing microbiome, and the immunoregulatory consequences of microbial encounter in barrier organs was the subject of the 99(th) Dahlem Conference.
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