A tentative earlier formulation1 noted that major infectious diseases of temperate zones seem to have arisen overwhelmingly in the Old World (Africa, Asia and Europe), often from diseases of Old World domestic animals. Hence one goal of this article is to re-appraise that conclusion in the light of studies of the past decade. Another goal is to extend the analysis to origins of tropical diseases3. We shall show that they also arose mainly in the Old World, but for different reasons, and mostly not from diseases of domestic animals.