This special focus chapter reviews the existing evidence on the extent to which extreme weather events can influence the burden of communicable diseases. The analysis adopts a One Health lens, which makes a strong case for collaboration and co‑ordination across human and animal health sectors, agri-food systems and the environment to address threats to population health.
The Economic Case for Pandemic Preparedness and Response
Beyond Lockdowns
Abstract
Key messages
Extreme weather events are becoming more frequent and intense
Extreme weather events such as heatwaves, heavy precipitation, droughts and associated wildfires and coastal flooding are becoming more frequent and intense across every geographic region, including the OECD and EU/EEA countries.
The rise in the frequency and intensity of extreme weather events helps create a more favourable environment for many diseases to flourish and expand their reach. Extreme weather events have been shown to aggravate 58% of all known communicable diseases that impact human health.
The rise in the frequency and intensity of extreme weather events influences changes in the landscape of communicable diseases
The health consequences of extreme weather events are already substantial and mounting. Beyond their immediate impact through heat-related illnesses, extreme weather events are reshaping the landscape of communicable diseases by
directly impacting the emergence, distribution and resurgence of many communicable diseases through their influence on the pathogens, vectors, hosts and the environment within which diseases are transmitted.
indirectly by exacerbating the increased risk of opportunistic infections and co-morbidities and malnutrition and limiting access to public health services and medicines, exacerbating food insecurities, altering migration patterns and increasing the likelihood of engaging in risky behaviours.
Mirroring global trends, there has been a sustained expansion of environmental suitability in the EU/EEA countries for major arboviruses over the last three decades, including dengue transmission by Aedes aegypti and by Aedes albopictus, chikungunya transmission by Aedes albopictus and Zika virus transmission by Aedes aegypti.
The risk of water-borne diseases rises with floods, heavy rainfall, hurricanes and heatwaves. Heavy precipitation, particularly following dry periods, has been shown to trigger outbreaks of cryptosporidiosis, giardiasis and other gastrointestinal illnesses, while major flooding events have caused large, well-documented outbreaks in Europe and North America. Increases in sea surface temperature and sea level are also expanding the environmental suitability for Vibrio species, heightening risks in coastal regions.
Heatwaves and heavy rainfall are increasingly recognised as important drivers of food-borne diseases. Rising water temperatures in freshwater and marine systems have mixed effects on food-borne pathogens, with some bacteria proliferating in warmer conditions and others showing higher survival at lower temperatures. Heavy rainfall also elevates contamination of surface waters and aquaculture systems, increasing risks from Campylobacter, Listeria and Escherichia coli (E. coli).
Key antimicrobial resistance (AMR)-causing organisms, including Campylobacter, E. Coli, Klebsiella pneumoniae and Staphylococcus aureus, have shown sensitivity to rising temperatures. As these pathogens already account for most AMR-related deaths in the OECD and EU/EEA countries, extreme weather events could intensify the future AMR burden.
Disease surveillance systems can be bolstered to counter risks associated with extreme weather events
The existing disease surveillance systems can be strengthened to consider the rising risks of extreme weather events by
Integrating data from multiple sources, including information on pathogens, hosts, vectors, human populations as well as meteorological and environmental conditions;
Risk mapping the environmental suitability for disease vectors;
Providing tools for early warning and targeted public health action;
Using predictive models that couple disease dynamics with meteorological and environmental data and
Strengthening early warning systems to integrate the high risk of extreme weather events.
Extreme weather events such as heatwaves, heavy precipitation, drought and associated wildfires and coastal flooding are becoming more frequent and intense across every geographic region, including the OECD and EU/EEA countries (ECDC, 2010[1]; IPCC, 2023[2]). For example, in 2024, Europe’s warmest year on record, persistent and stable drought conditions gripped Southern and Central and Eastern Europe, while heatwaves continued to become longer, more frequent and more intense (C3S and WMO, 2025[3]). Exceptional heat and drought have been exacerbating wildfire risk across Europe, North America and beyond. Heavier downpours have been triggering catastrophic flooding in many areas (C3S and WMO, 2025[3]).
The health consequences of extreme weather events and climate‑related events are already substantial and mounting. The European Environment Agency estimated that between 1980 and 2020, around 85 000 to 145 000 people lost their lives due to weather events across the EU/EEA countries, with more than 85% of these deaths occurring due to heatwaves (European Environment Agency, 2022[4]). More recent events have shown that the attributable risks remain high. In 2024 alone, storms and floods affected an estimated 413 000 people and resulted in at least 335 deaths across Europe (C3S and WMO, 2025[3]).
Beyond these immediate impacts, extreme weather events are reshaping the broader communicable disease landscape. A growing body of evidence shows that extreme weather events have already led to worrisome increases in the incidence and severity of weather-related vector-borne, water-borne and food-borne diseases; precipitated the emergence of infectious diseases with non-human animal origin (WHO, 2021[5]); contributed to the emergence and resurgence of diseases such as dengue, hantavirus and cholera (Wu et al., 2016[6]); led to shifts in the geographic distribution and seasonal patterns of diseases (McIntyre et al., 2017[7]) and contributed to the emergence of clustered disease outbreaks and outbreaks in non-traditional geographic areas and in unexpected times (Wu et al., 2016[6]).
Considering that extreme weather events are becoming more frequent, it is crucial to assess their links with communicable diseases. This chapter aims to examine the extent to which extreme weather events may exacerbate future pandemic risks by influencing communicable disease patterns. The existing evidence on extreme weather events and communicable diseases is reviewed, using methods grounded in the One Health approach. (Box 4.1). While the chapter attempts to be as comprehensive as possible in its scope, the drivers of changes in communicable disease patterns other than extreme weather events (e.g. domestic and international travel, urbanisation, socio‑economic development) are excluded. A review of the links between extreme weather events and their impact on population health other than communicable diseases is also outside the scope of the chapter. The chapter also does not assess how robust the systems for implementing non-pharmaceutical interventions are or the extent to which the general public complies with them in countries at higher risk of extreme weather events. The chapter concludes by discussing the ways in which disease surveillance systems can be enhanced to take into account the rising risks of extreme weather events.
The importance of One Health is widely acknowledged in tackling threats to population health
One Health is a multi-disciplinary and multi-sectoral approach to improving population health. It acknowledges that human health is closely interlinked with the health of animals, agri-food systems and the environment. It promotes adopting a collaborative framework that addresses the complex drivers of communicable diseases across multiple sectors while designing and implementing programmes, policies, legislation and research (WHO, 2022[8]).
Over the last two decades, the centrality of One Health has been increasingly recognised as a vital step to improve population health. For example, building on previous efforts, the WHO, the Food and Agriculture Organization of the United Nations and the World Organisation for Animal Health formed the Tripartite Alliance in 2010 to help operationalise the One Health approach (FAO, OIE and WHO, 2017[9]). More recently, the COVID‑19 pandemic, once again, spotlighted the importance of One Health. In 2021, many countries, including G20 countries, committed to strengthening the implementation of One Health in the Rome Declaration (Global Health Summit, 2021[10]). In 2022, the United Nations Environment Programme formally joined the Alliance on One Health.
The role of the environment within the One Health framework is often overlooked
To date, much of the effort to strengthen the implementation of One Health focussed on the intersection between human and animal health. In this light, WHO recently highlighted that the environment can broadly impact human health through three pathways:
Acting as a reservoir through which nutrients, living organisms (e.g. animals, plants, microorganisms), pathogens and chemicals are accumulated and transported
Serving as a focal point for ecological and chemical processes essential to human health
Acting as a health mediator through which disease agents in the environment are transferred to human and animal hosts either directly from the soil, air or through feeding or contact with infected organisms.
The WHO underscored the importance of exploring all three of these pathways to better understand the linkages at the human, animal and environment interface and underlined that it is crucial to assess how changes in the environment affect all these three mechanisms.
Source: WHO (2022[8]),Global Tuberculosis Report, https://www.who.int/teams/global-tuberculosis-programme/tb-reports/global-tuberculosis-report-2022; FAO, OIE, WHO (2017[9]), The Tripartite’s Commitment Providing multi-sectoral, collaborative leadership in addressing health challenges, www.oie.int/2010tripartitenote; European Commission and G20 (2021[10]), Global Health Summit – The Rome Declaration, https://www.g20.utoronto.ca/2021/Global_Health_Summit_Rome_Declaration.pdf.
Extreme weather patterns are already driving profound changes in communicable disease patterns. One recent systematic assessment found that 58% (218/375) of known communicable diseases that affect human health have been shown to be aggravated by changes in weather patterns and rising temperatures (Mora et al., 2022[11]).Another assessment found that in Europe, around 63% of human and domestic animal pathogens are sensitive to extreme weather events and other meteorological changes (McIntyre et al., 2017[7]). Table 4.1 provides examples of communicable diseases that have been shown to be sensitive to extreme weather events.
The remainder of this section provides an overview of the pathways through which changes in the environment and extreme weather events alter communicable disease patterns, with a focus on three disease categories: i) vector-borne diseases, ii) water-borne diseases and iii) food-borne diseases. An overview of the definitions used in the chapter is provided in Box 4.2.
Extreme weather conditions can influence the emergence, distribution and resurgence of many communicable diseases by influencing the pathogens, vectors, hosts and the environment within which diseases are transmitted. Definitions of some of the technical terms used in this chapter include the following:
Agent refers to infectious organisms, pathogens and chemical contaminants. In general, the presence of an agent is necessary but not sufficient for a disease to occur. A number of factors such as an organism’s ability to result in disease or dose of exposure influence whether the presence of an agent will cause disease
Pathogen refers to organisms (e.g. bacteria, viruses, parasites and fungi) that can cause diseases in human and animal populations. A range of factors influence whether exposure to a pathogen will generate disease (e.g. the ability of the pathogen to cause disease)
Vector refers to living organisms (e.g. ticks, fleas, mosquitoes) that act as an intermediary that carries a disease agent from a reservoir to a host that is susceptible to the disease
Host refers to humans or other living organisms that are susceptible to infectious agents. A host’s susceptibility and response to an agent is influenced by a number of factors including genetic composition, nutritional and immunologic status, anatomic structure, presence of disease or medications and psychological makeup
Environment refers to a host of external factors that bring together hosts and agents. Environmental factors encompass physical factors (e.g. geology), biological factors (e.g. the presence of vectors like insects to transmit agents) and socio‑economic factors (e.g. crowding, sanitation and accessibility of healthcare services)
Basic reproduction number refers to the average number of secondary cases that occur in a susceptible population after the introduction of one infectious individual. The value of gives information about the potential magnitude of the outbreak. Broadly, if the value of exceeds 1, an outbreak is likely to occur/persist. Conversely, if is below 1, then the outbreak is likely to end
Incubation period refers to the time elapsed between exposure to an infectious agent and the onset of disease
Vectorial capacity is a summary measure that tracks the capacity of a vector to transmit disease based on vector abundance, survival, competence and feeding rate and the length of the extrinsic incubation period
Climate suitability (CS) for infectious disease transmission is a measure that helps assess the extent to which shifts in temperature and precipitation can promote the transmission of infectious diseases. Specifically, rises in temperature and precipitation have been shown to increase the suitability of many vector- and tick-borne diseases. Changes in are often used in the literature to track trends in the CS of a number of pathogens such as dengue, chikungunya, Zika, malaria and Vibrio bacteria.
4.2.1. Extreme weather events are driving the spread of vector-borne diseases
Vector-borne diseases refer to illnesses caused by a wide range of parasites, viruses and bacteria that are transmitted via vectors (Table 4.2). In general, vector-borne diseases are considered to be limited to the geographic areas where their vectors are available. However, in recent years, rising temperatures and extreme weather events have been shown to precipitate an expansion in the distribution of many vectors in new geographic areas (Baylis, 2017[12]; El-Sayed and Kamel, 2020[13]).
Extreme weather events can drive complex changes in the transmission dynamics of vector-borne diseases by altering environmental conditions, vector ecology and host behaviour. Heavy precipitation, for example, can create favourable breeding environments for mosquitoes, particularly when combined with warming temperatures and elevated humidity. Evidence from the United States shows that such conditions can measurably increase disease risk (Soverow et al., 2009[15]). Using more than 16 000 WNV cases reported between 2001‑2005, one study found that cumulative weekly precipitation and the occurrence of at least one day of heavy rainfall in the preceding month were both associated with higher WNV incidence (Ibid). Shifts in precipitation patterns can also influence the movement and concentration of animal hosts, bringing them into closer proximity to vectors or human populations. For instance, rodent-borne hantavirus outbreaks have been linked to the displacement and aggregation of rodent populations following periods of intense rainfall or drought (Engelthaler et al., 1999[16]).
However, impacts of extreme weather events on vector ecology are not uniform such that some extreme conditions may have opposing or indirect effects. While drought can reduce the availability of natural mosquito habitats, it may simultaneously increase human-made breeding sites if households rely more heavily on stored water for domestic use (Rocklöv and Dubrow, 2020[17]). This dynamic has been documented in Northeastern Brazil, where increased household water storage during a prolonged drought created additional breeding habitats for Aedes aegypti, facilitating range expansion and elevating the risk of local dengue outbreaks (Pontes et al., 2000[18]).
Beyond vectors themselves, extreme weather events and changes in the environment are also reshaping the behaviour, distribution and interactions of hosts and broader ecosystems. Shifts in temperature and rainfall patterns have been linked to changes in the geographical range of reservoir species, enabling pathogens to emerge in areas previously unaffected. In Australia, for example, weather-related disturbances have contributed to the southward movement of black flying foxes (i.e. reservoirs for the Hendra virus) leading to spillover events in horses and subsequent human infections in new regions (Baker et al., 2021[19]).
Arboviruses
Arboviruses use vectors such as mosquitoes, ticks, midges and sandflies as their main transmission routes to cause infection and serious disease in humans (Table 4.3). For example, two types of mosquitoes (i.e. Aedes aegypti and Aedes albopictus) serve as vectors for diseases such as dengue, chikungunya and Zika virus. For certain arboviruses such as West Nile Virus (WNV), humans are considered to be a dead-end host such that the infected individuals do not contribute to onward transmission (Sigfrid et al., 2018[20]). For other arboviruses, such as dengue, chikungunya and Zika virus, humans are considered to be the main reservoir for infection, which poses a great risk of disease outbreaks without requiring an animal reservoir (ibid).
Source: Modified from (Sigfrid et al., 2018[20]), Preparing clinicians for (re‑)emerging arbovirus infectious diseases in Europe, https://doi.org/10.1016/j.cmi.2017.05.029.
Over the last three decades, there have been worrisome trends in the occurrence of disease outbreaks caused by arboviruses such as dengue and chikungunya across the OECD and EU/EEA countries as follows:
Since the 1927/28 outbreak in Greece, countries in Europe were largely spared from locally transmitted cases of dengue (Lillepold et al., 2019[21]). This trend changed in 2010 when Croatia and France reported locally contracted cases of dengue. Over the last decades, sporadic dengue outbreaks occurred in France between 2013‑2015 and in 2018 (ibid). More recently, Spain also reported locally transmitted dengue cases.
Similarly, chikungunya was not present in European countries until recently. Italy was the first country that experienced large chikungunya outbreaks in 2007 and 2017, with more than 200 confirmed and probable cases occurring in each outbreak (Rezza et al., 2007[22]). Other locally transmitted chikungunya cases have also been reported in France in 2010, 2014 and 2017, though the scale of these outbreaks was substantially smaller compared to Italy (Lillepold et al., 2019[21]).
The disease burden of the Zika virus remains low among OECD and EU/EEA countries (Rabe et al., 2025[23]). Costa Rica, Chile, Colombia, the United States of America and France are among OECD countries that have reported current or previous Zika virus transmission as of May 2024 (ibid). There were 22 reported cases of Zika in the EU/EEA in 2020, 7 in 2021 and 34 in 2022 (ECDC, 2023[24]; ECDC, 2024[25]). According to ECDC data, there were 79 travel-associated Zika cases in 2023 (Meredith, 2025[26]).
While the overall burden associated with these diseases remains low in the vast majority of OECD and EU/EEA countries, the rise in their burden is closely linked to increasing CS for the transmission of these diseases. Evidence from recent global analyses shows that climatic conditions have become progressively more favourable for the spread of arboviruses such as dengue, Zika and chikungunya over the past five decades (Romanello et al., 2022[27]). Using the basic reproduction number as a proxy for CS, one study estimated an increase of 11.5% in suitability for dengue transmission by Aedes aegypti, 12% for dengue by Aedes albopictus, 12% for chikungunya by Aedes albopictus and 12.4% for Zika virus transmission by Aedes aegypti between 1951‑1960 and 2012‑2021 (Ibid). Over the same period, the length of the transmission season for these arboviruses increased by approximately 6%, indicating both expanding geographical suitability and longer windows of potential transmission. Coupled with this, there has been a 6% increase in the length of transmission season for all arboviruses in this period (ibid).
Mirroring the global trends, there has been a steady long-term increase in the CS for the transmission of Zika, chikungunya and dengue over the last three decades in European countries (Figure 4.1). The suitability of Zika, measured using its basic reproduction number, shows the largest relative growth, rising from 0.14 in 1990 to 0.26 by 2022. Chikungunya suitability also increased, rising from 0.08 to 0.16 in the same period. CS for dengue transmitted by Aedes albopictus also rose gradually and consistently from 0.09 in 1990 to approximately 0.17 by 2022, while the suitability for dengue transmitted by Aedes aegypti also increased in the same period.
Note : Europe refers to the 38 member and co‑operating countries of the European Environment Agency.
Source: (van Daalen et al., 2024[28]), The 2024 Europe report of the Lancet Countdown on health and climate change: unprecedented warming demands unprecedented action, https://doi.org/10.1016/s2468-2667(24)00055-0.
Evidence suggests substantial cross-country variation in the CS for the transmission of many arboviruses. Across the EU/EEA members, the CS for the transmission of chikungunya is estimated to be the highest in countries in Southern and Central Europe including in Croatia, Hungary, Romania and Bulgaria. In contrast, Norway, Sweden, Finland and Denmark have the lowest CS for the transmission of this disease (van Daalen et al., 2022[29]). Projections of the geographic distribution of Aedes aegypti and Aedes albopictus point towards expanded distributional potential across much of Western Europe, the Balkan region with particularly high risk around the Mediterranean and Adriatic coasts (Ibid). (Semenza and Paz, 2021[30]) report that areas of France, Spain, Germany and Italy may experience increases in climatic suitability for chikungunya, particularly along major river basins such as the Rhine and Rhone. The abundance of mosquitoes is also expected to increase in the next two decades, suggesting that more individuals will be at risk of malaria and dengue fever (Baharom et al., 2021[31]).
The geographic expansion of vectors and viruses is a crucial factor that exacerbates the CS of arboviruses. The most invasive mosquito, known as the Asian tiger mosquito (i.e. Aedes albopictus) is now considered to be endemic across countries in Southern Europe (Sigfrid et al., 2018[20]). This vector was shown to cause the first chikungunya outbreak in the province of Ravenna in Italy in July-August 2007 (Rezza et al., 2007[22]). Similarly, Aedes albopictus has been shown to expand its range. For example, in 2017, eggs and larvae of Aedes albopictus were found in separate locations in the United Kingdom (Public Health England, 2017[32]). The Aedes aegypti mosquito, introduced to the island of Madeira in Portugal in 2005, caused the first locally transmitted dengue outbreak in 2012 (Seixas et al., 2013[33]). This was considered to be the largest dengue outbreak in Europe in recent history, with the number of dengue cases exceeding 2 000 (Sigfrid et al., 2018[20]). Similar trends can be observed in North America. For example, one study estimated that the average annual incidence of dengue in Mexico would increase around 12‑18% by 2030, 22‑31% by 2050 and 33‑42% by 2080 (Colón-González et al., 2013[34]).
Several factors have been shown to explain the links between rising temperatures and vectorial capacity as follows:
Rising temperatures can create a better environment for the development, survival and proliferation of many vectors. For example, higher temperatures have been associated with increases in the survival, abundance and feeding activity of the mosquitoes Aedes aegypti and Aedes albopictus (Reinhold, Lazzari and Lahondère, 2018[35]). Previous studies also showed that higher temperatures in several Northern European countries between 1950 and 2018 contributed to increased development rates for the tick Ixodes ricinus, the vector that causes Lyme disease, tick-borne encephalitis and Crimean – Congo haemorrhagic fever (Estrada-Peña and Fernández-Ruiz, 2020[36]).
Higher temperatures can also alter the activity patterns of mosquitoes and ticks. For example, several studies have associated the rise in WNV infections in Europe to higher temperatures. This can partially be explained by the virus’s ability to survive the winter in the mosquito vector, reduce the extrinsic incubation period and activate an earlier breeding season (Semenza and Paz, 2021[30]; Watts et al., 2021[37]; Young et al., 2021[38]; Chen et al., 2013[39]).
Higher temperatures can also lead to a longer transmission season and expansions in the geographic range of vectors (El-Sayed and Kamel, 2020[13]). For example, studies show that from 1951‑1960 to 2012‑2021, the number of months suitable for malaria transmission has increased by 31.3% in highland areas of the WHO region of the Americas (Romanello et al., 2022[27]). The spatial distribution of malaria cases has also been shown to expand to higher altitudes in highland regions of Colombia and Ethiopia partly due to increases in temperature over time (Siraj et al., 2014[40]).
In addition to the factors mentioned above, air pollution and in particular exposure to fine particulate matter (PM2.5) has been suggested to significantly increase the risk of respiratory infections and illnesses. For example, a recent systematic review concluded that a 10 μg/m3 increase in daily PM2.5 levels was associated with a 1.5% rise in influenza risk (Orr et al., 2025[41]).
Tick-borne diseases
Extreme weather events and broader changes in the environment are also influencing the distribution and activity of non-mosquito vectors, including Ixodes ricinus ticks, which transmit Borrelia burgdorferi (i.e. the causative agent of Lyme disease) and tick-borne encephalitis virus. Evidence suggests that these vectors are expanding into wider geographic regions over time (The Lancet Microbe, 2021[42]; Couper, MacDonald and Mordecai, 2021[43]). In North America, rising temperatures have been associated with an earlier onset of the Lyme disease transmission season in the United States, as well as the northward movement of key reservoir hosts such as the white‑footed mouse into previously unsuitable areas of Canada (Rocklöv and Dubrow, 2020[17]).
Similar patterns are observed in Europe. Multiple studies indicate that warmer temperatures have contributed to shifts in both the seasonal activity and geographic range of Ixodes ricinus, increasing the potential for Lyme disease transmission across Northern and Central European regions (Semenza and Paz, 2021[30]; Lindgren, Tälleklint and Polfeldt, 2000[44]; Estrada-Peña and Fernández-Ruiz, 2020[36]).
The distribution of other vector species such as sandflies, which transmit leishmaniasis, are also shifting (Rocklöv and Dubrow, 2020[17]). Warmer temperatures and shifting humidity patterns have already altered the geographic footprint of Phlebotomine sandflies, with evidence that future changes could support their expansion into central and Northern Europe, including Great Britain and Scandinavia (Semenza and Paz, 2021[30]). Such shifts imply that leishmaniasis, historically concentrated in Southern Europe, may emerge in regions previously considered low risk.
4.2.2. Extreme weather events alter the burden of water-borne diseases by impacting the quality, quantity and distribution of water resources
Water-borne diseases are closely linked with extreme weather events. Specifically, the forecasted increases in the frequency and intensity of floods and heavy rainfalls, driven primarily by the increases in the sea surface temperature, and more frequent hurricanes and El Niño events, are expected to increase the burden of water-borne diseases such as giardiasis, cryptosporidiosis, infections with pathogenic Escherichia coli (E. coli), Shigella, cholera, viral hepatitis A and enteric/diarrhoeal diseases (El-Sayed and Kamel, 2020[13]). These extreme events can compromise drinking-water systems, facilitate the spread of pathogens through runoff and sewage overflows and increase human exposure to contaminated water sources.
Rising temperatures further compound these risks. A substantial body of evidence indicates that higher temperatures are associated with increased incidence of water-borne diseases (Levy et al., 2016[45]; Levy, Smith and Carlton, 2018[46]). A systematic review and meta‑analysis examining the relationship between temperature and diarrhoeal disease found a positive association between increases in mean temperature and all-cause diarrhoea (incidence rate ratio [IRR] 1.06; 95% CI: 1.03‑1.09), with a similarly elevated risk for bacterial diarrhoea (IRR 1.07; 95% CI: 1.04‑1.10) (Carlton et al., 2015[47]). Evidence from OECD countries is consistent with these findings. In Australia, weeks with maximum temperatures above 31°C were associated with a 13.6 percentage points (p.p.) increase in the risk of cryptosporidiosis (Lal et al., 2013[48]). In Canada, analysis of 92 water-borne outbreaks between 1975 and 2001 found that outbreak risk increased during warmer weather (Thomas et al., 2006[49]).
Rising temperatures can also alter human exposure and susceptibility to infections by influencing their behaviour and engagement in activities but the existing evidence remains mixed (Committee on Emerging Microbial Threats to Health in the 21st Century, 2003[50]). For example, warmer weather can increase engagement in recreational water-related activities, which, in turn, increases the risk of water-borne diseases (Wu et al., 2016[6]). Extreme temperatures might also cause people to spend more time indoors, thereby promoting human-to-human transmission (Mora et al., 2022[11]).
Increases in the frequency and intensity of heavy rainfalls also precipitate a rise in water-borne disease outbreaks. For example, in the United States, one study that used data on 548 water-borne outbreaks between 1948‑1994 found that more than half of outbreak events were preceded by heavy precipitation events within two months (Curriero et al., 2001[51]). Heavy precipitation events that follow a dry period have also been linked to increased risk of water-borne diseases. For example, one study used data collected from 2009 to 2014 from 4 cities whose water systems rely heavily on surface waters of the North American Great Lakes (e.g. Lake Ontario and Lake Michigan) (Graydon et al., 2022[52]). This study concluded that the risk of acute gastrointestinal illnesses caused by cryptosporidiosis and giardiasis increased after extreme precipitation events in 3 out of the 4 cities, with the relative risk estimates ranging roughly between 1.1 and 1.5 in 3‑5 weeks after extreme precipitation weeks.
Flooding events can increase the risk of water-borne disease outbreaks by undermining the existing water supply systems and sanitation infrastructure. For example, one study from the city of Halle in Germany studied the potential impact of extreme river flooding after a heavy rain event in 2013. The study demonstrated that the extreme river flooding impacted the sewage system across the city and resulted in the contamination of the river water and water in recreational areas (Gertler et al., 2015[53]). In turn, the contamination of these water sources led to the largest cryptosporidiosis outbreak, a diarrhoeal disease caused by a parasite called Cryptosporidium, in Germany to date. The outbreak started about 6 weeks after the peak of the river flooding and led to the emergence of 167 cryptosporidium cases. Another study from the capital city of Copenhagen in Denmark studied the effects of an unusually heavy rainfall that occurred in August 2010 the day before a triathlon competition, which led to severe flooding and overflow in the sewer system (Harder-Lauridsen et al., 2013[54]). This study found that this extreme weather event increased the risk of illnesses caused by Campylobacter, Giardia lamblia and diarrhoeagenic E. coli among swimmers that were exposed to the contaminated seawater during the triathlon compared to swimmers who were not. Other evidence from Austria, Bulgaria and France also highlight that floods are associated with increases in the incidence of leptospirosis, a re‑emerging bacterial disease caused by exposure to contaminated water (Suk et al., 2020[55]).
Hurricanes and tsunamis can also trigger a rise in water-borne disease outbreaks. For example, in 2005 Hurricane Katrina, a category 4 storm, made landfall on the coast of Louisiana in the United States, particularly affecting the city of New Orleans. In the aftermath of the hurricane, an outbreak of acute gastroenteritis caused by norovirus occurred among more than 1 000 evacuees and relief workers in a temporary shelter at Reliant Park in Houston, Texas (Yee et al., 2007[56]). In India, the Indian Ocean tsunami that occurred in 2004 was followed by an outbreak of diarrhoeal illnesses caused by rotavirus in a temporary shelter (Liang and Messenger, 2018[57]).
Increased sea surface temperature can influence the life cycle of pathogens. One recent assessment showed that changes in sea surface temperature and surface salinity led to an expansion in the suitability for Vibrio bacteria (i.e. the disease agents that are found in marine waters and cause a wide range of infections in humans such as gastroenteritis, wound infections, sepsis and cholera). Specifically, one recent study suggested that around 86.3% of the Baltic coastlines have become suitable for infections caused by non-cholera Vibrio bacteria between 2014‑2021 (Romanello et al., 2022[27]). This is a substantial rise from about 47.5% in 1982‑1989 (ibid). In the United States between 2014‑2021, about 57.1% of the Northeast coastlines became environmentally suitable for non-cholera Vibrio infections, a substantial rise compared to about 30% in 1982‑1989 (ibid). The CS of infections caused by the non-cholera Vibrio bacteria also increased in the Pacific Northwest in the same period albeit at a smaller rate from 1.2% to 5.7% (ibid). This study further concluded that the proportion of global coastal waters suitable for Vibrio cholera has also expanded by 3.5% since 2003-2005 (Romanello et al., 2022[27]).
Rising sea levels have also been suggested to influence the burden of water-borne diseases by raising groundwater tables, damaging water and sanitation infrastructures, contaminating drinking water sources and facilitating the transfer of microbial pollutants from terrestrial sources (e.g. sewage) into marine environments used for food harvesting and recreational activities (National Collaborating Centre for Environmental Health, 2022[58]). Sea level rise can also create more suitable conditions for the proliferation of Vibrio by influencing the salinity of water (Jacobs et al., 2015[59]).
4.2.3. Extreme weather events are increasingly recognised as important drivers of food-borne diseases
Emerging evidence suggests that extreme weather events are an important driver of food-borne diseases, which can affect the entire food safety chain, from primary production and processing to storage and distribution.
Extreme heat exposure can lead to the proliferation of food-borne illnesses caused by bacteria such as Salmonella. One study from the United States extracted data from the Food-borne Diseases Active Surveillance Network between 2004‑2014 and found that extreme heat exposure was associated with a 7% increase in the risk of Salmonella infections in Maryland and a 6% increase in Tennessee (Morgado et al., 2021[60]). This study also pointed to a worrisome rise in the risk of Salmonella serotype Javiana infections in Connecticut and Georgia that was associated with extreme precipitation events.
Similar findings have been reported in other OECD countries. One study used data from 1991 to 2001 from the National Notifiable Disease Surveillance System, including Perth, Adelaide, Melbourne, Sydney and Brisbane (D’Souza et al., 2004[61]). It showed that a 1‑degree Celsius increase in temperature in the previous month was associated with a 4.1% and 11% increase in the cases of salmonellosis in Perth and Brisbane respectively. One study from 10 European countries found that a 1‑degree Celsius rise in weekly temperature was associated with a 5‑10% increase in the number of salmonellosis cases (Kovats et al., 2004[62]), whereas another study projected that outbreaks in temperature‑related incidences of Salmonella are expected to rise in Europe (Akil, Anwar Ahmad and Reddy, 2014[63]).
Rising surface water temperatures in fresh and marine water systems can have a mixed impact on the burden of food-borne illnesses. To date, some studies suggest a positive association between water temperature and the incidence of Salmonella and Campylobacter (Hellberg and Chu, 2015[64]) whereas others suggested an inverse relationship. For example, according to a recent study by (Semenza and Paz, 2021[30]), incidence of Campylobacteriosis is expected to increase by almost 200% across Scandinavian countries by the end of the century, which would translate to nearly 6 000 excess campylobacter cases per year (Ibid). Another study examined data collected from 21 sites in the coastal areas of Bahia de Todos Santos in Mexico between 2004 and 2006 (Simental and Martinez-Urtaza, 2008[65]). This study found that there was an inverse association between temperature and the detection of Salmonella. The authors indicated that this finding may be explained by the potential impact of sunlight and very high levels of solar radiation on the survival of the bacteria. Another study from Canada used data from the Grand River watershed in southern Ontario (Cheyne et al., 2009[66]) to show that two Yersinia enterocolitica virulence genes, a Gram-negative bacterium that causes a food-borne disease called yersiniosis, was more frequently detected in colder water temperatures. Another study from the United States also reported an inverse correlation between water temperature and detection frequency of Campylobacter in a rural mixed-use watershed over 2 years (2007‑2009), though the study did not find a significant association between water temperature and detection frequency of Salmonella (Vereen et al., 2013[67]).
Heavy precipitation events are likely to increase the burden of food-borne illnesses by increasing the risk of water contamination. One study from Germany demonstrated that during and after heavy rainfall events, run-off from sewer overflow and non-point sources was associated with higher levels of Campylobacter spp. (Rechenburg and Kistemann, 2009[68]). Another study from Germany found that elevated levels of C perfringens and E coli in tributaries of multiple drinking water reservoirs were observed after extreme rainfall and runoff from surrounding agricultural and forest lands (Kistemann et al., 2002[69]). A subsequent study from Finland found that levels of Listeria spp. found in fish farms were elevated after periods of heavy rainfall (Miettinen and Wirtanen, 2006[70]). This study argued that this finding was most likely due to contaminated brooks and rivers, as well as other runoff waters in nearby areas.
Extreme weather events and rising temperatures can also impact pathogens directly by altering the habitats that pathogens and their competitors can survive in. For example, Campylobacter spp., Gram-negative bacteria that causes food-borne Campylobacter infections, has been shown to prosper in surface waters at low temperatures and during winter months (Jones, 2001[71]). The higher concentration of bacteria can partly be explained by the fact that warmer temperature can help promote the proliferation of bacteria that compete with Campylobacter spp.
4.2.4. Many pathogens that cause resistant infections are sensitive to extreme weather events
Antimicrobial resistance (AMR) remains a leading global health challenge that jeopardises the effectiveness of many medical and public health advances made in the 20th century. Patients with resistant infections have a greater risk of developing complications, a lower probability of recovery and a greater risk of death. Treating resistant infections tends to be costlier than treating susceptible infections because more intensive medical procedures and more aggressive antimicrobial therapies are generally used to treat these infections.
In 2023, the OECD produced a comprehensive assessment of the health and economic burden of AMR in 51 OECD, EU/EEA and G20 countries (OECD, 2023[72]). This publication demonstrated that in excess of 4.3 million resistant infections occurred annually across 34 countries included in the analysis. Approximately 65% of these infections were estimated to be acquired in community settings. The analysis also showed that up to 32.5 million extra days were spent in hospitals to treat resistant infections every year across the countries included in the OECD analysis. Considering this, the OECD and EU/EEA countries were estimated to spend up to USD 28.9 billion annually to treat resistant infections.
Findings from the latest OECD analysis suggested that extreme weather events are likely to exacerbate the AMR burden (OECD, 2023[72]). Many pathogens that cause significant AMR infections and deaths are sensitive to extreme weather events. For example, Campylobacter, a pathogen sensitive to climate, is responsible for more than 1.5 million infections that occur in community settings every year across 34 countries included in the OECD analysis, corresponding to around 36% of all resistant infections. Similarly, previous evidence showed that the incidence of superbugs such as E. coli, Klebsiella pneumoniae and Staphylococcus aureus have been shown to be sensitive to rising temperatures (Kaba, Kuhlmann and Scheithauer, 2020[73]). This poses a substantial threat to population health because the OECD analysis showed that these three superbugs are responsible for around three‑quarters of all AMR-related deaths that occur annually across the 34 countries included in the analysis.
4.2.5. Extreme weather events can impact other diseases through indirect channels
An emerging strand of literature suggests that the effects of extreme weather events and rising temperatures can also impact communicable disease patterns indirectly. For certain diseases, such as tuberculosis (TB) and human immunodeficiency virus (HIV), the links between the changes in the environment and disease patterns may look less direct. For example:
A growing body of evidence suggests that changes in temperature, precipitation, humidity and wind speed influenced the transmission of TB through the metabolic functions of Mycobacterium tuberculosis (i.e. the causative agent of TB) (Gelaw et al., 2019[74]; WHO, 2025[75]). For example, a study in Japan found that both extreme heat and cold temperature events were associated with a considerable rise in TB cases, with the estimated relative risks of 1.2 and 1.23 respectively (Onozuka and Hagihara, 2015[76]).
Increases in temperature have also been linked with increased burden of HIV by increasing the risk of co-morbidities (Hewitt et al., 2006[77]; Guinto et al., 2022[78]) and HIV-associated opportunistic infections (Tong et al., 2017[79]), limiting access to public health services and antiretroviral medications (Lieber et al., 2021[80]), exacerbating food insecurities (Austin, Noble and Berndt, 2020[81]), altering migration patterns and increasing the likelihood of engaging in risky behaviours (e.g. substance use) (Baker, 2020[82]; Bellandi, 2022[83]; Vergunst et al., 2022[84]).
OECD, EU/EEA and G20 countries generally operate advanced infectious disease surveillance systems, comprising routine notification mechanisms, laboratory networks and emergency alert platforms such as the EU’s Early Warning and Response System (EWRS). While effective for detecting many conventional outbreaks, these systems were not designed with accelerating meteorological extremes in mind, increasing the risk that communicable diseases that are sensitive to extreme weather events and changes in the climate go undetected or recognised only once they have already escalated.
Several limitations constrain the ability of current surveillance systems to detect and respond to health risks amplified by extreme weather:
Routine health surveillance and weather information systems continue to operate largely in parallel. Only 23% of ministries of health (MOHs) globally report using meteorological data within their health surveillance systems, even though 74% of national meteorological and hydrological services provide relevant meteorological data (WMO, 2023[85]). Use is even lower for diseases sensitive to extreme weather events (ibid). Only 14% of MOHs incorporate meteorological information into vector-borne disease surveillance and only 12% do so for water-borne diseases and other water-related health outcomes (ibid).
Current disease surveillance systems remain centred on human health, with hospital-based reporting serving as their primary foundation (Shen et al., 2025[86]). Because hospital-based systems detect cases only after individuals seek care, they could identify outbreaks with significant delays. For example, when COVID‑19 struck, only a handful of countries (i.e. Denmark, Estonia and Latvia) had the infrastructure to have near real-time data on key areas such as hospital in-patients, emergency care, primary care, long-term care and prescription medicines (Oderkirk, 2021[87]).
Surveillance networks tend to focus on a predefined list of notifiable pathogens, which may lead to under-detection of diseases that fall outside this list. Heatwaves, for example, may drive sporadic cases of uncommon infections (e.g. Vibrio wound infections in unusually warm waters or fungal respiratory infections after wildfires) that are outside the list of notifiable diseases. This was the case in Northern European countries with access to the Baltic Sea, where a substantial spike in vibriosis cases was observed after a heatwave during the summer of 2018 (Amato et al., 2022[88]).
Surveillance capacity also varies widely across countries. While some countries maintain well-established surveillance infrastructure, others face persistent barriers ranging from limited financial resources and insufficient laboratory capacity to fragmented data-sharing arrangements (Shen et al., 2025[86]; Han et al., 2023[89]). In turn, these limitations delay timely detection.
Disparities also exist within countries. Extreme weather events tend to disproportionately affect rural and remote communities (Dewi et al., 2024[90]) and socio‑economically disadvantaged communities (Birkmann et al., 2022[91]). These populations often face longstanding barriers to healthcare access and rely on infrastructure and essential services that are more easily disrupted by extreme weather events such as floods, storms or heatwaves. As a result, in such settings, routine surveillance systems may not operate reliably, increasing the likelihood that outbreaks remain undetected or are captured only after significant delays.
4.3.2. Building infectious disease surveillance systems that consider the rising burden of extreme weather events requires comprehensive action
As extreme weather events increasingly influence communicable disease patterns, OECD, EU/EEA and G20 countries will need to modernise their surveillance systems to better anticipate and manage emerging risks. A central priority is the integration of multi-source data, including information on pathogens, hosts, vectors, human populations, meteorological and environmental conditions (Shen et al., 2025[86]).
Some OECD countries are already using multiple data sources to inform their disease surveillance efforts. For example, in the United States, the surveillance of WNV is based on two complementary approaches. Epidemiological surveillance focusses on human infections to measure the attributable health burden, detect early outbreak signals and inform timely action (CDC, 2024[92]). This includes analysing seasonal, geographic and demographic patterns in human mobility and mortality (ibid). Environmental surveillance, on the other hand, monitors local mosquito populations, viral activity in mosquito vectors and non-human hosts (e.g. birds) and other relevant environmental parameters (ibid). This surveillance helps predict human infection risk and enables preventive actions to avert outbreaks in people.
Another important step is the risk mapping of environmental suitability for disease vectors. Mapping current and projected meteorological conditions can highlight where vectors such as mosquitoes or ticks could thrive, which in turn indicates where diseases might emerge or spread. In Canada, for instance, one project conducted model-based risk assessments to anticipate vector-borne disease emergence (WMO, 2023[85]). The first step was to identify which geographic regions were likely to be affected and when new risks were likely to appear. This was done by developing risk maps under current and future scenarios (ibid).
Providing tools for early warning and targeted public health action is equally important. For example, in Europe, the Lancet Countdown initiative included a risk assessment framework in its 2022 report (Alcayna, Rao and Lowe, 2025[93]). This effort included indicators to track the climatic suitability of several infectious diseases such as dengue, malaria, non-cholera Vibrio and West Nile virus.
The use of predictive models that couple disease dynamics with meteorological and environmental data can also help account for the impacts of extreme weather events. In Canada, for example, mathematical models of vector life cycles and disease transmission have been used to assess the climatic limits for various vectors and the diseases they carry (WMO, 2023[85]). These models shed light on how meteorological variables affected each stage of a vector’s life and the pathogen’s replication, while accounting for other ecological and socio‑economic factors that influence disease occurrence. This approach yielded notable early warnings. For example, projections of the northward expansion of the blacklegged tick (i.e. the Lyme disease vector) under warming scenarios proved highly accurate and successfully predicted the emergence of Lyme disease in previously unaffected Canadian regions (ibid).
Beyond routine surveillance, countries can also strengthen EWRSs to better anticipate infectious disease outbreaks in the context of extreme weather events. Many of the infectious diseases detected by EWRSs are highly sensitive to extreme weather events. These systems can be further strengthened by incorporating meteorological information (WHO, 2021[94]). Several EWRSs in OECD, EU/EEA and G20 countries already integrate meteorological information (e.g. temperature, precipitation, extreme weather indicators) with epidemiological data to anticipate disease risks (Box 4.3). For example, in Mexico, one study showed that incorporating meteorological data into the national dengue and Zika surveillance accurately predicted outbreaks, with alarm signals predicting a dengue outbreak with 100% accuracy and a Zika outbreak with 97% accuracy (Cardenas et al., 2022[95]).
The European Union’s EWRS, operational for more than two decades, serves as a central platform for monitoring communicable disease trends and identifying emerging health risks across EU members (European Commission, 2025[96]). It integrates information on approximately 50 communicable diseases, as well as other health issues such as AMR to enable public health authorities to rapidly assess threats and co‑ordinate interventions that may require cross-border co‑operation. Historically, the EWRS has supported responses to major events such as pandemic influenza A(H1N1), COVID‑19 and, more recently, monkeypox (European Commission, 2025[96]).
The system is activated under four conditions:
When a threat is unusual or unexpected for a given place and time;
when it results in substantial morbidity or mortality;
when it exceeds national response capacity; or
when it affects multiple EU countries and may require co‑ordinated action at the Union level.
Contingent on the responsiveness of Member States, alerts are issued within 24 hours of an EU country or the Commission becoming aware of a potential threat. For instance, the first notification of what would later be identified as SARS‑CoV‑2 on 9 January 2020 was issued just days after China reported the emergence of an unidentified viral pneumonia (European Commission, 2025[96]).
Unlike specialised surveillance systems that track specific pathogens, the EWRS is designed to detect a broad range of hazards, including biological, chemical, environmental and unexplained events. This all-hazards mandate is increasingly important in the context of extreme weather events, because the EWRS also monitors extreme weather events and other environmental disruptions that may amplify infectious disease risks (European Commission, 2025[96]). For example, the system tracks severe floods, heatwaves, volcanic ash clouds, chemical spills and biological hazards such as biotoxins. Recently, the EWRS was activated for dengue outbreaks in the EU and overseas territories following a spike in cases across Italy, France and Spain in 2022 (European Commission, 2024[97]). As a result, disease surveillance at airports and ports was expanded to curb the spread of infections (ibid).
Source: European Commission (2025[96]); “Surveillance and early warning”, https://health.ec.europa.eu/health-security-and-infectious-diseases/surveillance-and-early-warning_en#the-early-warning-and-response-system-ewrs; European Commission (2024[97]), “E-000791/2024 Answer given by Ms Kyriakides on behalf of the European Commission”, https://www.europarl.europa.eu/doceo/document/E-9-2024-000791-ASW_EN.pdf.
Extreme weather events and rising temperatures are reshaping the epidemiology of communicable diseases. These shifts are already evident across OECD and EU/EEA countries, affecting vector-, water- and food-borne diseases and heightening the risk of antimicrobial resistance. The rising risks associated with extreme weather events underscore the need for strengthening disease surveillance platforms that integrate environmental, meteorological and epidemiological data to detect emerging threats earlier and safeguard population health.
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