Hygiene can hurt.

I pass on this summary from the Editor's choice section of Science, follwed by the abstract of the work mentioned. It describes further work on how failure to interact with natural environments during childhood can lead to later chronic inflammatory disorders. Also, relevant to this topic is a recent Op-Ed piece in the NYTimes called "Dirtying Up Our Diets.", and this further piece discusses our human microbiome.

As human societies urbanize, chronic inflammatory disorders become more apparent. The hygiene hypothesis suggests that individuals exposed to infection in childhood are less likely to develop inflammatory disease because exposure to microorganisms is important for stimulating responses that maintain epithelial cell integrity. Hence, in urban environments, reduced contact with the full diversity of the microbial world may be leading to the increased incidence of inflammatory disorders. Hanski et al. took a random sample of 118 adolescents from towns, villages, and isolated dwellings in eastern Finland, tested their immune function and allergic responses, surveyed their skin microflora, and investigated the biodiversity within their homes. They found several significant correlations, not least that low biodiversity was surprisingly strongly associated with atopy, and concluded that humans need to interact with natural environments for their physical health, not just for their peace of mind.

Here is the Hanski et al. abstract:

Rapidly declining biodiversity may be a contributing factor to another global megatrend—the rapidly increasing prevalence of allergies and other chronic inflammatory diseases among urban populations worldwide. According to the “biodiversity hypothesis,” reduced contact of people with natural environmental features and biodiversity may adversely affect the human commensal microbiota and its immunomodulatory capacity. Analyzing atopic sensitization (i.e., allergic disposition) in a random sample of adolescents living in a heterogeneous region of 100 × 150 km, we show that environmental biodiversity in the surroundings of the study subjects’ homes influenced the composition of the bacterial classes on their skin. Compared with healthy individuals, atopic individuals had lower environmental biodiversity in the surroundings of their homes and significantly lower generic diversity of gammaproteobacteria on their skin. The functional role of the Gram-negative gammaproteobacteria is supported by in vitro measurements of expression of IL-10, a key anti-inflammatory cytokine in immunologic tolerance, in peripheral blood mononuclear cells. In healthy, but not in atopic, individuals, IL-10 expression was positively correlated with the abundance of the gammaproteobacterial genus Acinetobacter on the skin. These results raise fundamental questions about the consequences of biodiversity loss for both allergic conditions and public health in general.