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. The goal is simple: recalibrate the beneficial bacteria in a person’s gut to fight off the ones that are causing them harm.

It’s the loss of these beneficial microbes that’s thought to cause the diarrhoea in the first place. Our guts are teeming with bacteria, which outnumber our own cells and