Rising city populations face unique challenges.

• Position: Urban Overcrowding

The World Health Organization identifies the rapid increase of urban populations, especially slum populations, as the most important issue affecting health in the 21st century. The agency cites overcrowding, lack of safe water, and improper sanitation systems as the primary factors contributing to poor health among the urban poor. Slums often become breeding grounds for diseases like tuberculosis, dengue, pneumonia, and cholera, and slum dwellers contract water-borne or respiratory illnesses at much higher rates than people in rural areas do.

The world's urban population is expected to grow by 2,600,000,000 by 2050, bringing the total number of urbanites to 6,300,000,000, according to research conducted by the Worldwatch Institute, Washington, D.C. This urban expansion especially will be burdensome for developing countries, where 82% of the world's population currently lives, writes report author Grant Potter.

Although the developing world is less urbanized than the industrial world in relative terms, developing countries are home to an estimated 1,540,000,000 more people. In absolute terms, the developing world is projected to add approximately 2,450,000,000 people to its cities by 2050, while the industrial world is due to add 170,000,000.

Within the developing world, the vast majority of this urban growth is projected to occur in Asia and Africa. Asia far outstrips Africa in total population--5,000,000,000 people to 1,000,000,000--but these regions also are the least urbanized areas on...