An influx of European settlers and African slaves to the Greater Caribbean after 1620 steered ecological change onto new paths. In particular, changes took place that made the region especially hospitable to the mosquito vectors of yellow fever and malaria. The next two chapters outline those ecological changes, the habits of the relevant mosquitoes, the character of those two diseases, and the ways in which people tried, almost always fruitlessly, to cope with yellow fever and malaria.