Population Growth, Density, and Distribution

Until recently, most of the world's population lived in rural areas. In 1800, for example, less than 1.7 percent of people lived in urban communities. By 1970, however, more than a third of the world's people lived in urban settings. By the year 2000, that fraction is expected to rise to one-half (Dentler, 1977).

Not only are more people choosing to live in urban areas, but the size and density of many cities are also increasing, in part because of the overall population growth rate—each year the population of the world grows by approximately 70 million. High birth rates in many cities contribute to urbanization. By the end of the century, there will be 425 cities with a million or more inhabitants, an increase of 200 cities since 1985 (United Nations, 1985; World Resources Institute, 1986). Twenty-five cities are expected to have populations that exceed 11 million (Last and Wallace, 1992).

In many parts of the world, urban population growth has been accompanied by overcrowding, poor hygiene, inadequate sanitation (including wastewater disposal), and insufficient supplies of clean water. Urban development, with its attendant construction, emergence of slum areas and shanty towns, and infrastructure needs (e.g., water treatment and waste disposal facilities), has also caused ecological damage. These factors have created conditions under which certain disease-causing organisms and the vectors that carry them have thrived. The dengue viruses and their primary mosquito vector, Ae. aegypti, are one such example.

DENGUE

There are four distinct serotypes of dengue virus, each of which can cause a spectrum of illnesses ranging from mild fever and general malaise (dengue fever) to shock and fatal hemorrhagic disease (dengue hemorrhagic fever/dengue shock syndrome [DHF/DSS]). Dengue typically is a disease of young children, although older children and adults can be affected. Dengue viruses are transmitted to humans by Ae. aegypti mosquitoes.

Although dengue fever has plagued tropical populations for hundreds of years, the more severe form of the disease, DHF/DSS, is relatively new. The first recognized epidemic of DHF/DSS occurred in Manila in 1953 (Hammon et al., 1960). Dengue fever is usually the result of primary infection with one of the four dengue virus serotypes. DHF/DSS occurs in people who have been infected with two or more serotypes. The global spread and mixing of dengue serotypes have been made possible by the movement of infected individuals from one area to another.

Over the past 15 years, outbreaks of dengue fever have become increasingly numerous and severe, especially in urban centers in the tropics. At the



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