Climate variability and climate change are affecting infectious-disease transmission patterns in multiple ways. For example, diseases traditionally associated with tropical and subtropical regions are reaching new areas of the world. Rising temperatures and precipitation are making temperate, northern or mountainous countries more susceptible to outbreaks of "southern" or “low land” diseases like malaria. Nepal, previously too cool for dengue fever, suffered its first outbreak in 2006, with a handful of cases. Since then, the incidence of dengue has increased significantly. Before 1970, dengue fever caused severe outbreaks in only nine countries. Now it is endemic in more than 100 countries, according to the World Health Organization.
A projected increase in the frequency and intensity of disasters associated with climate change could displace a growing number of people. The World Bank estimates that, in three regions alone, there will be 140 million internally displaced people by 2050 due to climate change. As people migrate, they not only place substantial demands on the ecosystems and social infrastructures where they move, but also carry illnesses that emerge from shifts in infectious-disease vectors.
Figure 1 provides a framework on infectious disease transmission types, including human to human, animal to animal, and animal to human. Climate change is increasing the global emergence, resurgence and redistribution of infectious diseases risks across all of these.