The expansion of personal horizons mentioned above has implications for the whole of society: people’s geographic knowledge increases exponentially the further they ride or walk.
If you travel up to twenty miles from home, you can potentially see four times as many places than if you only ever travel ten miles. It is a matter of mathematical radius: the twenty-mile traveller can theoretically cover 1,257 square miles, as opposed to the 314 square miles of a ten-mile traveller. Moreover, if everyone regularly travels twenty miles from home, rather than just ten, they will meet many more people from further away, with similarly extensive geographical knowledge.
In theory, a man who travels twenty miles from home could meet someone who lives twenty miles beyond that. If that second person has previously travelled twenty miles from his home in the opposite direction, the first person might here a first hand report of a place sixty miles away from his home town.
That theoretically covers an area of more than 11,000 square miles. Obviously, the difficulties of negotiating rivers, mountains, moors and coastlines means things aren’t quite that simple. But the point is clear: when people travel further, their collective knowledge increases exponentially.
With hundreds of thousands of people moving further and further from home as the Middle Ages progressed – in some cases hundreds of miles – knowledge about England and other countries increased massively. This expansion of collective horizon helped to spread geographical confidence as people had a higher level of prior knowledge of what they might encounter when away from home.
This confidence in turn allowed them to venture even further.Ian Mortimer – Medieval Horizons [amazon]
Mortimer doesn’t specifically discuss implications of the same network effect on other areas of knowledge here, but the same principle applies to the spread of ideas in general.
And, presumeably, to diseases.