Disease in Africa, as elsewhere, has been and continues to be intimately linked with the ways that human populations have fed themselves. Throughout most of the evolutionary journey of Homo sapiens on the African continent (again, as elsewhere), the species existed in small bands of hunter-gatherers with generally fewer than 100 members. As such, individuals were constantly on the move, seldom pausing in one place long enough to foul their water supplies or let their garbage and excrement pile up. They were, as a consequence, spared a host of waterborne parasites and insect disease vectors. Because hunter-gatherers did not have domesticated animals, they were also spared the incredible number of ailments that animals pass along to human masters. Moreover, their numbers were too small to support directly transmitted microparasitic diseases such as smallpox and measles. Finally, evidence suggests that hunter-gatherers were essentially free from the predominant noncommunicable diseases of today, such as cancer, heart-related diseases, and diabetes. This does not mean that they enjoyed perfect health. They were tormented by a variety of arthritic conditions and suffered from accidents connected with hunting and warfare. Those living within the line of 40-inch rainfall would have been infected from time to time by the trypanosome, which is presumably a very ancient parasite of wild animals. When it is transmitted to humans and large animals by the tsetse fly, it causes often-deadly sleeping sickness. Indeed, as William McNeill points out, the disease probably set some limits on the territory in which early humans were able to hunt and gather.