Global climate change has increased the occurrence of some infectious diseases. Those infectious diseases whose transmission is impacted by climate change include for example the vector-borne diseases dengue fever, malaria, tick-borne diseases, leishmaniasis, zika fever, chikungunya and Ebola virus disease. One of the mechanisms for increased disease transmission is that climate change is altering the geographic range and seasonality of the insects (or disease vectors) that can carry the diseases. Scientists stated a clear observation in 2022: "the occurrence of climate-related food-borne and waterborne diseases has increased (very high confidence)."
Infectious diseases that are sensitive to climate can be grouped into: vector-borne diseases (transmitted via mosquitos, ticks etc.), waterborne diseases (transmitted via viruses or bacteria through water), and food-borne diseases. Climate change is affecting the distribution of these diseases due to the expanding geographic range and seasonality of these diseases and their vectors. Like other ways in which climate change affects on human health, climate change exacerbates existing inequalities and challenges in managing infectious disease.
Mosquito-borne diseases that are sensitive to climate include malaria, elephantiasis, Rift Valley fever, yellow fever, dengue fever, Zika virus, and chikungunya. Scientists found in 2022 that rising temperatures are increasing the areas where dengue fever, malaria and other mosquito-carried diseases are able to spread. Warmer temperatures are also advancing to higher elevations, allowing mosquitoes to survive in places that were previously inhospitable to them. This risks malaria making a return to areas where it was previously eradicated.
Ticks are changing their geographic range because of rising temperatures, and this puts new populations at risk. Ticks can spread lyme disease and tick-borne encephalitis. It is expected that climate change will increase the incidence of these diseases in the Northern Hemisphere. For example, a review of the literature found that "In the USA, a 2°C warming could increase the number of Lyme disease cases by over 20% over the coming decades and lead to an earlier onset and longer length of the annual Lyme disease season".
Waterborne diseases are caused by a pathogen transmitted through water. The symptoms of waterborne diseases typically include diarrhea, fever and other flu-like symptoms, neurological disorders, and liver damage. Changes in climate have a large effect on the distribution of microbial species. These communities are very complex and can be extremely sensitive to external climate stimuli. There are a range of waterborne diseases and parasites that will pose greater health risks in future. This will vary by region. For example, in Africa Cryptosporidium spp. and Giardia duodenalis (two protozoan parasites) will increase. This is due to increasing temperatures and drought.
Scientist also expect that disease outbreaks caused by vibrio (in particular the bacterium that causes cholera, called vibrio cholerae) are increasing in occurrence and intensity. One reason is that the area of coastline with suitable conditions for vibrio bacteria has increased due to changes in sea surface temperature and sea surface salinity caused by climate change. These pathogens can cause gastroenteritis, cholera, wound infections, and sepsis. The increasing occurrence of higher temperature days, heavy rainfall events and flooding due to climate change could lead to an increase in cholera risks.
Public health context
In 1988, little was known about the effects of climate change on human health. As of 2023, the evidence has grown significantly and is for example summarised in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. The scientific understanding about the potential health risks and observed health impacts caused by climate change is now better understood. One category of health risks is that of infectious diseases. A study concluded in 2022 that "58% (that is, 218 out of 375) of infectious diseases confronted by humanity worldwide have been at some point aggravated by climatic hazards". The World Health Organization considers climate change as one of the greatest threats to human health.
Infectious diseases have played a significant role in human history, impacting the rise and fall of civilizations and facilitating the conquest of new territories. During recent decades, there are significant regional changes in vector and pathogen distribution reported in temperate, peri‐Arctic, Arctic, and tropical highland regions.