regions of the world. Plasmodium falciparum, the most clinically dangerous of the malaria parasites, is most widespread in Africa south of the Sahara and throughout the world 's tropics. Plasmodium ovale is found almost exclusively in Africa. Plasmodium malariae has the same geographic range as P. falciparum, although it is much less prevalent and occurs in more restricted zones.
An estimated 2.8 billion people—nearly 60 percent of the world's population—live in areas free from malaria, either because the disease never existed there, because it disappeared as an unintended consequence of human activities, such as road building and urbanization, or because it was eliminated intentionally through eradication efforts. Another 1.7 billion, or slightly more than 30 percent of the population, live in areas where the malaria incidence declined because of eradication efforts but now is making a comeback. Malaria transmission has remained essentially unchanged for 9 percent of the world's inhabitants, about 445 million people. Most in this latter group live in Africa south of the Sahara.
The distribution of malaria within and between geographic regions is greatly influenced by the human population itself. Biological factors such as immune response and genetics, as well as socioeconomic status, living and working conditions, exposure to vectors, and human behavior, all play a critical role in determining a person's risk of infection and illness (Figure 2-7).