Exposure to diverse microbes could explain why children who grow up on farms are less likely to develop asthma than their suburban counterparts.

Previous work showed that children raised on farms are protected from childhood asthma and a class of allergic reactions called 'atopy'. Now, Markus Ege of the University Children's Hospital Munich in Germany and his colleagues have analysed the microbial populations in dust collected from 933 children's rooms. They found that bacteria and fungi were more numerous and widespread in samples collected for children who live on farms. They also found that the risk of asthma and atopy decreased as the number of microbial taxa increased. In particular, fungi from two genera, Eurotium and Penicillium, were tightly associated with reduced asthma risk.

N. Engl. J. Med. 364, 701–709 (2011)