spread of diseases, including influenza (IOM, 2005); severe acute respiratory syndrome (SARS; IOM, 2004); drug-resistant malaria (IOM, 2003; Martens and Hall, 2000); and chikungunya² in Europe (Angelini et al., 2007). Indeed, it is possible to travel between most places in the world in less time than the incubation period for many infectious diseases (Wilson, 2003), as was illustrated in spring 2009 by the rapid, global spread of the new, swine origin, influenza A (H1N1) virus (Dawood et al., 2009; Khan et al., 2009).

Travel is not only becoming increasingly rapid and more socially wide-spread, but is also more ubiquitous. Travelers and tourists connect once-remote areas, which serve as both “sources” and “sinks” for emerging infectious diseases, to more developed regions. International trade in food and other agricultural commodities, as well as in wildlife, has also increased markedly among an ever-widening network of producers and markets. Pathogens accompany live animals, plants, and their byproducts across continents and oceans; microbes and vectors also hitch rides in ballast water³ and in shipping crates and containers. Upon arrival in industrialized countries, such as the United States, potentially disease-containing goods can be redistributed nationwide within hours.

Travel and trade have been linked with disease since antiquity. People instinctively feared and isolated ill travelers long before the causative agents of infectious diseases were known or described (Gushulak and MacPherson, 2000). Quarantine laws, established to prevent the importation of plague—without success—in fourteenth-century Venice, were eventually adopted throughout Europe and Asia (Fidler et al., 2007; Markel et al., 2007).

International endeavors to contain infectious diseases commenced more than 150 years ago and are today embodied in the International Health Regulations (IHR), which provide the legal framework for global cooperation on infectious disease surveillance (IOM, 2007; Stern and Markel, 2004). While ideally there are strong incentives for nations to support global efforts to address infectious disease threats, such efforts have from their outset been characterized by a lack of authority for enforcement and weak inducements for participation (Stern and Markel, 2004).

More subtly, but no less importantly, introduced animals, plants, and microbes can disrupt ecosystems in ways that increase the potential for infectious disease outbreaks. Such changes can be more difficult to predict than the movements of pathogens, and more daunting to prevent. The term “invasive species” is widely used to describe plants and animals that spread aggressively when introduced to and established in new environments freed from the constraints found in their native environments (Dybas, 2004). Given both the similarities and characteristics of such invasions with those of pathogenic microbes, it may prove fruitful to view the origins of disease emergence, establishment, and spread through the