Climate restricts the range of infectious diseases, while weather affects the timing and intensity of outbreaks. The ranges of several key diseases or their vectors are changing, along with shifts in plant communities and the retreat of alpine glaciers. In addition, extreme weather events associated with warming create conditions conducive to ”clusters” of disease outbreaks. The rapid spread of West Nile virus in the Americas is related, paradoxically, to drought and its impact on wildlife (230 species of animals, 138 species of birds) could alter the ratios of predator birds to their prey (including rodents) and thus have implications for human health and agriculture. Moreover, the same global changes and genetic exchanges are contributing to the emergence of pests and pathogens in livestock, crops, trees and coral reefs. Emerging diseases have thus become an additional force of global change and pose new threats to habitat and 'Nature's services.' Advances in climate forecasting and satellite imaging can help generate early warning systems, which can catalyze timely, environmentally-friendly public health interventions. If climate warming continues to be associated with more volatility and more intense and frequent weather extremes, we have begun to see the profound social consequences and costs of an unstable climate.