The hygiene hypothesis and multiple sclerosis

Epidemiological studies indicate that the industrially developed world is characterized by a high and increasing burden of allergic and autoimmune diseases. For example, Holgate considers allergy a pandemic and points out that currently almost half of the populations of developed countries suffer from atopy, asthma, or another allergic condition. In the case of autoimmunity, Jacobson and colleagues estimate that 3 to 5% of the US population suffers from a serious immune-mediated disease such as insulin-dependent diabetes, inflammatory bowel disease, connective tissue disease, or multiple sclerosis (MS). In addition, Bach summarizes evidence suggesting that in the last 50 years the incidence of allergic and autoimmune diseases has increased dramatically, and the most persuasive supporting data for this trend come from conditions such as insulin-dependent diabetes, where acute treatment is life-saving and diagnosis is straightforward. In developing countries, by contrast, allergy and autoimmunity are uncommon. Clearly, this dichotomous epidemiological pattern requires a scientific explanation. One account put forth to explain the observed global distribution and temporal changes in the incidence of allergy and autoimmunity is the hygiene hypothesis. Yazdanbakhsh and colleagues summarize the hygiene hypotheses as follows: