In 1992, the Institute of Medicine's report, Emerging Infections : Microbial Threats to health in the United States, expanding international travel and commerce was identified as one of six principal factors contributing to the global development and spread of emerging and reemerging pathogens. International travel has increased dramatically during the 20th century, and populations are now in motion to a degree never before seen in history. It has been estimated that during 1995 over 1.4 tourists corssed international borders every day, an annual total of over 500 million persons. Mobile populations include leisure and business travelers, military personnel, immigrants, refugees and missionaries. Travelers not only put themselves at risk by visiting areas where diseases are emerging, but they also run the risk of exposure to disease they do not usually encounter at home. After exposure they may then incubate these illnesses and not manifest symptoms until they return home, risking the potential for spread of microbes to new areas. International travelers visiting developing countries are at particular risk for a variety of bacterial, viral, and parasitic infectious diseases. Increased trade and expanded markets for imported foods, which occasionally contain bacterial or viral contaminants. International interchange of mankinds and goods afford ample opportunity for the unrecognized movement of pathogens from place to place and for rapid global spread of microbial agents. From the microbe's point of view, the global village of the late 20th century provides global opportunities for disease emergence and transmission.