“Everyone on this planet is separated by only six other people”. So says a character in John Guare’s 1990 play Six Degrees of Separation. It’s a maxim that has come to define our ideas about the reach of social networks – and there’s some truth in it too. The average number of friends-of-friends connecting you to any other random person might not be exactly six – it depends on how you define links, for one thing – but it is a small number of about that size.

But has it always been such a small world? It’s tempting to think so. Jazz musicians in the early 20th Century were united by barely three degrees of separation. Much further back, scientists in the 17th Century maintained a dense social network via letters, as did humanist scholars of the Renaissance. But those were specialised groups. Intellectual and aristocratic elites in history might have all known one another, but was it always a small world for ordinary folk too? Studying such social networks is difficult, because the relationships of the average person living in pre-industrial times were rarely documented. Yet there could be an indirect way to find out – by studying the spread of disease.