Five hundred years ago, the spread of disease was largely constrained by the main mode of transportation of the time: people traveling on foot. An outbreak in one town would slowly ripple outward with a pattern similar to what occurs when a rock drops onto a surface of still water. The Black Death moved across 14th century Europe in much this way, like concentric waves unfurling across the continent.
Today, disease migrates across populations and geography with a curiously different pattern. In 2003, SARS first appeared in China, then spread to Hong Kong, then turned up from there in Europe, Canada and the United States. Plot the spread of the disease on a map of the world, and its movement looks downright random.