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For example, more people travel more often over greater distances and in less time now than at any time in the past. One consequence of the increased mobility in the modern age can be seen in the 2003 outbreak of the novel illness SARS, which rapidly spread from Hong Kong to Toronto and elsewhere as infected passengers traveled by air.

To better understand and predict disease emergence, Dr. Morens and his coauthors stress the need for research aimed at broadly understanding infectious diseases as well as specifically understanding how disease-causing microorganisms make the jump from animals to humans.

In a narrow sense, epidemics are caused by particular microorganisms, and the study of infectious disease has historically been microbe-focused. For example, the Black Death (bubonic plague), which killed some 34 million Europeans in the middle of the 14th century, was caused by the bacterium Yersinia pestis. In a broader sense, however, epidemics are caused by complex and not fully predictable interactions between the disease-causing microbe, the human host and multiple environmental factors, the authors note. The Black Death, for instance, was borne westward along newly established land and sea trade routes from its probable origin, China, into multiple European countries. Similarly, patterns of human movement along trade routes, specifically truck routes throughout Africa, played a role in the spread of HIV throughout that continent. Greater consideration must be given, say the NIAID authors, to broader, interlinked factors such as climate, urbanization, increased international travel and the rise of drug-resistant microbes, and the ways in which these factors combine to spark new epidemics.

Aside from commerce and travel, the NIAID authors point to several other factors that underlie many notable emerging diseases: poverty, the breakdown of public hygiene practices, and susceptibility of human populati
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