Global climate change is expected to contribute to the worldwide burden of disease and premature deaths. Scientists predict that rising average temperatures in some regions will change the transmission dynamics and geographic range of cholera, malaria, dengue fever, and tick-borne diseases. Increased precipitation may raise the number and productivity of breeding sites for vectors such as mosquitoes, ticks, and sandflies. An interim report released in 2015 by the National Climate Assessment asserts that climate change will influence the distribution, abundance, and rate of infection of mosquitoes, increasing human exposure to bites from infected insects. Rising atmospheric and surface temperatures are also increasing the intensity, frequency, and duration of heavy precipitation and flooding events, which may raise the risk of diarrheal diseases and vector-borne infections such as Lyme disease. Furthermore, new vector-borne pathogens are likely to emerge as a result of climate change; they have already been seen in shifting disease patterns of mosquito-borne diseases such as dengue fever, West Nile virus, and chikungunya. 

A number of diseases—for example, malaria, dengue fever, and viral encephalitis—are highly sensitive to changes in the environment. The 1993 outbreak of hantavirus pulmonary syndrome (an acute respiratory disease) in the Southwest United States provides a dramatic example of how severe weather events can promote disease transmission. The outbreak was traced to a steep increase in the population of deer mice that carry the virus—an increase caused by heavy rains after 6 years of drought, which led to an abundance of food sources for the deer mice. 

 

Lyme disease has also become more prevalent in the United States. Its incidence has doubled since 1991, when the disease was first detected. New Hampshire and Vermont have experienced the largest increases, followed by Delaware, Maine, and Massachusetts. Although there are multiple reasons for the increase in Lyme disease cases, recent studies suggest that climate change contributed to the expanded range of ticks. It is now possible that ticks may be able to survive in Canada, leading to the possibility of Lyme disease in a country previously not at risk. 

 

Several recent studies have examined the relationship between the occurrence of infectious diseases and short-term climate variation—in particular, the influence of the El Niño–Southern Oscillation (ENSO) cycle on the transmission of vector-borne infections (malaria, dengue) and non-vector-borne infections (cholera). ENSO is the irregular cycling of surface water temperatures between warm and cool phases across parts of the Pacific Ocean. Global climate change is expected to intensify ENSO-related climate variability.

 

Scientists are currently debating the future effects of climate change on vector-borne diseases. Some experts predict that many vector-borne diseases will expand their range to higher elevations and latitudes in response to global warming; others claim that human impacts on the local ecology—deforestation and water use and storage, for instance—have a far stronger influence on the frequency and range of vector-borne infections. Most agree that both are important to monitor as disease patterns change. In an effort to gain a better understanding of the relative effect of sustainable development practices and climate change on the health of local populations, scientists create model scenarios using these factors as variables. 

 

A secondary effect of climate change also promotes infectious disease. Human migration often follows extreme weather or weather-associated events, including hurricanes, cyclones, fires, drought, and floods. The risk of disease outbreaks increases after such disasters due to population displacement, resulting in unsafe food, water, and housing; crowding; and poor access to health care.