The fifteenth-century encounter between previously separate disease environments was not simply an encounter between Europe and the Americas. It was preceded by an encounter between Europe and tropical Africa and followed by a still-more-intense transmission of diseases across the tropical Atlantic, mainly from Africa to the Americas. This transmission principally involved smallpox, yaws, yellow fever, and falciparum malaria. Many other vector-borne diseases failed to make the transfer for lack of suitable vectors in the Americas. The African diseases contributed even more than those from Europe to the depopulation of the humid tropics in the Americas. They also set up conditions that made residence in the tropical Americas dangerous for newly arrived people from Europe.