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The aggregate human impact on the environment now exceeds the limits of absorption or regeneration of various major biophysical systems, at global and regional levels. The resultant global environmental changes include altered atmospheric composition, widespread land degradation, depletion of fisheries, freshwater shortages, and biodiversity losses. The drive for further social and economic development, plus an unavoidable substantial increase in population size by 2050 - especially in less developed countries- will tend to augment these large-scale environmental problems. Disturbances of the Earth's life-support systems (the source of climatic stability, food, freshwater, and robust ecosystems) will affect disproportionately the resource-poor and geographically vulnerable populations in many tropical countries. Ecological disturbances will alter the pattern of various pests and pathogens in plants, livestock and humans. Overall, these large-scale environmental changes are likely to increase the range and seasonality of various (especially vector-borne) infectious diseases, food insecurity, of water stress, and of population displacement with its various adverse health consequences.