When the first hominins and their successors migrated north from Africa into Eurasia, they created a new, interlinked disease environment. They brought some diseases, such as malaria, with them from Africa, and newly encountered others, such as plague, in Eurasia. Regional changes in climate played a role in human health, not simply due to their influence in determining the success of year-to-year harvests and grazing lands, but also because periods of warming or severe and sudden cooling shifted the interactions between humans and the flora and fauna that made up their environment. Exchanges of disease between the two continents would continue up through the medieval era. Whereas vast distances and low population density likely shielded Eurasian populations from frequent epidemic outbreaks up through the Neolithic period, by the beginning of the common era, with its vastly intensified trade networks, Eurasia would begin to see a new phenomenon: pandemics, including the Justinianic Plague and the Black Death, the largest mortality events in human history. The diseases of medieval Eurasia are still among the world’s leading infectious killers and causes of debilitating morbidity. Because they have all persisted to the present day (with the exception of smallpox), modern science plays an important role in their historical reconstruction.