World history has seen many such epidemics, some of which have drastically altered the fortunes of human societies. Studies in historical epidemiology by scholars like William H. McNeill and Alfred W. Crosby show that epidemics have been most destructive when pathogens have made their way to societies with large, previously unexposed populations. This has happened frequently when individuals from an infected society have had dealings with others from unexposed societies. During the third century ce, for example, merchants introduced exotic pathogens to societies along the silk roads and caused epidemics in Han China, the Roman empire, and probably other societies as well. After the fourteenth century, bubonic plague traveled the trade routes and ravaged societies throughout much of the eastern hemisphere. From the sixteenth to the nineteenth centuries, a massive transfer of biological species took place following the European voyages of exploration that inaugurated sustained interaction between the peoples of the eastern hemisphere, the western hemisphere, and Oceania. As a part of this larger “Columbian exchange,” epidemic smallpox, measles, diphtheria, and other diseases exacted a devastating toll from indigenous peoples in the Americas and Oceania. Indeed, these massive transoceanic epidemics may have caused more deaths than any other agent in human history.