The key to a hidden kingdom

It’s a modern pastime to hypothesise about what makes a good relationship. One evening not long ago in a Berlin bar, I listened to a friend diagnose how things were going with his partner: ‘We might have become a bit too symbiotic.’ Surprisingly earnest perhaps, but that’s what you get when a sociologist dates a psychoanalyst. On the way home, I wondered why symbiosis, apart from the obvious dangers of parasitism, might not be that desirable coexistence all our theories point toward. After all, the OED recommends that this ‘interaction between two different organisms living in close physical association’ is ‘typically to the advantage of both’. The darker side of symbiosis, I suppose, is the risk of losing yourself.

Yet the richest, riskiest symbioses are playing out inside you. The human body is home to 100 trillion microbes in mobile constellation (a mere 100 million stars, by comparison, make up the Milky Way).