Urbanization and tropical health--then and now.

Since the launch of the Annals of Tropical Medicine and Parasitology 100 years ago, the percentage of the world's population living in urban settings has more than tripled and is now approaching 50%. Urbanization will continue at a high pace, particularly in the less developed regions of Africa and Asia. The profound demographic, ecological and socio-economic transformations that accompany the process of urbanization have important impacts on health and well-being. In industrialized countries, urbanization led to the so-called 'epidemiological transition', from acute infectious and deficiency diseases to chronic non-communicable diseases, many decades ago. In the developing world, surprisingly little research has been carried out on the health-related aspects of urbanization. In a temporal analysis of publications in the Annals of Tropical Medicine and Parasitology, for example, in which the first volume in every decade from 1907 was examined, only 16 (2.6%) of the 604 articles investigated focused on epidemiological and/or public-health issues in urban tropical settings. This review begins with the question 'what is urban?' and then provides a summary of the trends seen in urbanization, and its impacts on human health, over the past century, on both a global and regional scale. For the main tropical diseases, estimates of the at-risk populations and the numbers of cases are updated and then split into urban and non-urban categories. The inhabitants of urban slums are particularly vulnerable to many of these diseases and require special attention if internationally-set targets for development are to be met. Heterogeneity, a major feature of urban settings in the tropics that complicates all efforts at health improvement, is demonstrated in an exploration of a densely populated municipality of a large West African town. Urban planners, public-health experts and other relevant stakeholders clearly need to make much more progress in alleviating poverty and enhancing the health and well-being of urban residents, in an equity-effective and sustainable manner.