By the year 2015, 59 percent of the world's population will live in urban centers. In the developing world, where one third of the urban population currently lives in slums or shantytowns, cities will need to absorb more than two billion new residents in the next three decades. But the pace of urbanization far exceeds the rate at which basic services can be provided, and the health consequences for the urban poor have been dire.

Contaminated water, inadequate sanitation, overcrowding, substandard housing, and poor hygiene have made urban slums home to the spread of infectious and parasitic diseases such as cholera, malaria, hepatitis, and tuberculosis. High rates of acute and chronic respiratory disease in developing world cities can be directly linked to air pollution from industry and heavy traffic as well as to indoor air pollution created by stoves that burn fossil fuels in poorly ventilated households. HIV/AIDS also has spread exponentially in cities due to poverty, high population density, and migration flows making it one of the most serious public health problems confronting cities in the developing world.

In addition to traditional diseases of poverty, urban populations in the developing world suffer disproportionately from "new" chronic and degenerative diseases of modernization, such as cardiovascular disease, hypertension, and cancer brought on by changing lifestyles and environmental stress. Meanwhile, social fragmentation and alienation have contributed to increased mental illness as well as threatening rates of crime and violence.

Health statistics often mask the severity of conditions for the urban poor. When disaggregated, data reveal sharp urban health inequalities and differences within levels of access to services. Diseases virtually absent in the official city are epidemic in squatter settlements.

As urban areas continue to grow, their sustainability will depend upon public policies that transcend traditional sectoral approaches, linking health initiatives to improvements in urban environmental conditions. With the support of the Urban Programs Team of the U.S. Agency for International Development, the Comparative Urban Studies Project is commissioning research on the complex nature of urban development with a focus on strategies to alleviate poverty. Together with practitioners, policymakers, and researchers, the Project is working to identify innovative approaches that incorporate urban issues, including health, into the broader development agenda.