Certain bacteria protect against a disease that is a growing threat
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Can you be too clean? That is the question posed by the hygiene hypothesis, which seeks to explain why, as many illnesses have become rarer in rich countries, some have become more common. The hygiene hypothesis posits that the rise of several of these diseases, including asthma, eczema and type-1 diabetes (all of which seem associated with malfunctions of the immune system), has been caused by improvements in hygiene of the sort that have helped get rid of other illnesses. Exactly how that might happen is unclear. But at the AAAS meeting Brett Finlay of the University of British Columbia, in Vancouver, persuasively filled in some of the blanks in the case of asthma.