into one giant breeding ground for microbes. Millions of Roman citizens were killed between 165 and 180 AD when smallpox finally reached Rome during the Plague of Antoninus. Three centuries later, the bubonic plague hit Europe for the first time (542–543 AD) as the Plague of Justinian. It returned in full force as the Black Death in the fourteenth century, when a new route for overland trade with China provided rapid transit for fleainfested furs from plague-ridden Central Asia.

Even before the development of world trade routes, however, human pathogens had experienced two major bonanzas. First, when people lived as hunter-gatherers, they were constantly on the move, making it difficult for microbes to keep up with their human hosts. Once people started living as farmers, they began residing in larger numbers in the same place—and were in daily contact with their accumulating feces—for extended periods of time. Second, the advent of cities brought even larger numbers of people together under even worse sanitary conditions. In the Middle Ages, when people threw human waste out their windows in England, they were said to be “blessing the passerby.”

Now, two millennia later, human pathogens are experiencing yet another bonanza from a new era of globalization characterized by faster travel over greater distances and worldwide trade. Although some experts mark the fall of the Berlin Wall as the beginning of this new era, others argue that it is not so new. Even a hundred years ago, at the turn of the nineteenth century, the tremendous impact of increased trade and travel on infectious disease was evident in the emergence of plague epidemics in numerous port cities around the world. As Echenberg (2002) notes, plague epidemics in colonial African cities were closely tied to the increased communication, travel, and trade that accompanied the advent of the steamship. The economic and social impacts of these epidemics were profound. In Johannesburg, in what is now South Africa, the occurrence of plague led to the relocation of black residents in an effort to remove what the white colonists believed was the source of the disease. At about the same time, the influenza pandemic killed many millions of people worldwide.

Thus the current era of globalization is more properly viewed as an intensification of trends that have occurred throughout history. Never before have so many people moved so quickly throughout the world, whether by choice or force. Never before has the population density been higher, with more people living in urban areas. Never before have food, animals, commodities, and capital been transported so freely and quickly across political boundaries. And never before have pathogens had such ample opportunity to hitch global rides on airplanes, people, and products.

The future of globalization is still in the making. Despite the successful attempts of the developed world during the course of the last century to control many infectious diseases and even to eradicate some deadly afflic-



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