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Extreme weather events are becoming more intense, and are likely to become more frequent as the world climate changes. 1,2 For epidemiologists, one important aspect of these trends is their impact on infectious disease.In this issue of Epidemiology, Woodruff et al. 3 demonstrate a strong association between heavy rainfall and outbreaks of Ross River virus disease. Ross River virus is mosquito borne, and, although the disease is little known outside of Australia, is found in nearly all parts of the continent. Infection can produce disabling polyarthritis, and there is no specific treatment. In southeast Australia, Ross River virus disease poses a threat not just to local populations, but also to tourism—Australia’s largest industry.The work of Woodruff et al. 3 adds to a growing body of studies 4 that explore the impact of weather on infectious disease. Beginning in the mid-1970s, there has been a worldwide emergence, resurgence, and redistribution of infectious diseases. 5 Although the spread of infectious diseases is multicausal, 6 global climate change may be a major contributor. Weather and climate can influence host defenses, vectors, pathogens, and habitat. Studying climate effects requires integration of information from many sources. For example, by mapping multiple datasets into geographic information systems (GIS), researchers can detect emerging patterns, and these spatial and temporal associations can lead to new hypotheses regarding causality.The findings of Woodruff et al. 3 are plausible. Although Ross River virus disease can spread even without excessive rain, extreme rainfall increases the likelihood of large outbreaks (via the mosquito Culex australicus). As floodwaters abate, oxbow lakes are pinched off at river bends, creating “billabongs”—ideal sites for breeding mosquitoes. With alert public health monitoring, preventive measures can be taken to avoid spread of the disease.But a larger question is, can we predict the conditions that lead to the spread of the disease? Our capacity for long-term weather forecasting has greatly improved, with monitoring of Pacific Sea surface temperatures and the state of the El Niño/Southern Oscillation. Such forecasts have helped predict weather patterns that may lead to infectious disease outbreaks. 7–9 There are longer trends that must also be considered in making weather projections—which brings us to the issue of climate change.In the study of global climate change, the field is an N of 1, an experiment that cannot be repeated. 10 For the past 420,000 years, atmospheric CO2 levels (which parallel average global temperatures) have remained within a range of 180 to 280 parts per million. 11 CO2 is now close to 370 parts per million and rising. 2 Taken as a whole, the Holocene Age—the past 10,000 years—has been one of the most stable periods in climate records. 12 Even so, warming this century has been rapid, and it is not occurring uniformly. Although the overall rate of maximum temperature increase since 1950 is approximately 1°C per century, minimum (nighttime and winter) temperatures have increased at twice that rate (∼2°C per century), 13 and winter temperatures are rising even faster near the poles. 2 In global change research, models are used to project what might be expected with warming, and data are examined for consistency with the projections. The accumulating number of such “fingerprint” studies forms the basis for the conclusion that human-induced climate change has indeed begun.Global warming has implications for the spread of infectious disease. Small arthropods are highly temperature sensitive, 14 and temperature constrains the range of vector-borne diseases like Ross River virus disease. 15–17 Ticks have been moving northward in Sweden as winters warm.