Eat Dirt — The Hygiene Hypothesis and Allergic Diseases

There has been an epidemic of both autoimmune diseases (in which the immune response is dominated by type 1 helper T [Th1] cells, such as type 1 diabetes, Crohn's disease, and multiple sclerosis) and allergic diseases (in which the immune response is dominated by type 2 helper T [Th2] cells, such as asthma, allergic rhinitis, and atopic dermatitis), as documented in the article by Bach in this issue of the Journal. 1 The occurrence of these diseases is higher in more affluent, Western, industrialized countries. One theory proposed to explain this increase in the prevalence of autoimmune and allergic diseases is . . .