Hygiene hypothesis: the notion that an almost obsessive emphasis on cleanliness in Western cultures has caused an increase in such autoimmune disorders as allergies and asthma.

In the past 50 years, asthma rates in the Western world have increased as much as threefold, and growing numbers of people have developed allergies. Since the late 1980s, studies have suggested a link to stricter hygiene standards, which have prevented children from frequently interacting with bacteria, fungi and other microbes. According to this hypothesis, exposure to a wide range of microbes early in life somehow conditions the immune system against asthma and allergies.

This symbiotic effect was documented most recently in a report published in The New England Journal of Medicine. German researchers examined the number and diversity of microorganisms to which 16,500 city and farm children were exposed, and discovered that a greater diversity of microbes correlated with lower asthma risk. James Gern, author of an accompanying editorial and former chair of the American Board of Allergy and Immunology, sees “promise that we‘ll be able to identify treatments based on these observations.”