Trade Routes and Disease

[A] The overland routes that carried silk westward carried other things as well, and the most consequential of them were microbial. A disease needs a chain of susceptible hosts, and a chain that would have broken in a sparsely populated landscape held together along a road where caravans met at the same wells and slept in the same caravanserais. [B]

[C] The pattern is visible in the record. Periods of intense long-distance trade correlate with outbreaks at both ends, sometimes within a few years of each other, and the correlation is too consistent to be accidental. What it does not show is a simple direction of travel: a route is not a one-way pipe, and the same road that delivered a plague to one region delivered a different one back. [D] It is also worth resisting the tidy conclusion that trade was therefore a net harm. The same routes carried the crops, the techniques and the texts on which the receiving societies later depended, and the historian who counts only the deaths is doing arithmetic on one column. Neither column can be filled in with much confidence, which is the deeper difficulty.