urbanisation: smaller families; a move from muddy countryside to concrete cities; a preference for chlorinated water and sanitised food; and a growing distance from livestock, pets and other animals. All of these changes have been consistently linked to a higher risk of allergic and inflammatory diseases, and all of them reduce the range of microbes that we are exposed to. A single dog can have a huge effect. When Susan Lynch hoovered up the dust of 16 homes, she found that those without furry pets were ‘microbial deserts’. Those with cats were far richer in microbes, and those with dogs were
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