Via ScienceNow: Roads and Floods Help Plague Spread. Excerpt:
Until now, studies of plague movements have typically focused on the local development of the infection over time, rather than the social and environmental conditions that allow the bacteria to jump between sometimes far-flung geographical locations.
In the new study, researchers from China and Norway addressed this underexplored issue. Armed with detailed Chinese historical records, they built a model that traced the origin and geographical spread of the third plague pandemic, which killed at least 10 million people between 1855 and 1959.
The researchers were able to reconstruct the plague’s transmission routes across China, they report online today in the Proceedings of the Royal Society B. After an initial outbreak in Yunnan, the disease advanced in three phases. The first and third began with local transmission events, with new outbreaks occurring close to old ones, but the second appears to have started when the bacteria made long-distance jumps to areas in the north and southeast of the country, hundreds of kilometers away from Yunnan.
The team noted that the plague tended to spread along established transportation lines, such as major roads, rivers, and coastlines, and moved fastest on these routes. Furthermore, heavy rains and floods increased the speed at which plague reached new areas. The researchers believe that such flooding may have forced people—and rodents—to move, bringing the disease with them.