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People swap bacteria all the time—through sneezes and coughs, through hugs and sex, or through touching the same surfaces. Then there are people who swap bacteria because a doctor deliberately transplants faeces from one into the other.

This isn’t a bizarre medical perversion. It’s usually a life-saving gambit. Faecal transplants—which are exactly what they sound like—are used to treat people who suffer from intense diarrhoea following rounds of antibiotics.