# 02 - 474 Climate Change and Infectious Disease

## 474 Climate Change and Infectious Disease

Government actions/public policy shifts
Effects
Impact on human health
Establish sustainable dietary guidelines
Eliminate subsidies for commodity crops
Add cost of environmental degradation to
foods with high environmental footprint
Reduce demand for beef
Reduced meat consumption
Shift to more plant-based diets
Make improvements in urban design
Eliminate subsidies for fossil fuels
Increased use of public transportation
Impact on climate change
Reduced greenhouse gases
FIGURE 473-8  Strategies to mitigate the impact of climate change on human nutrition. (Reproduced with permission of Journal of Clinical Investigation from Climate 
change and malnutrition: we need to act now. WH Dietz 130:556, 2020.)
■
■INFECTIONS AND DIARRHEAL DISEASE
 (See Chap. 474.)
POTENTIAL SOLUTIONS
Reductions in GHG emissions will require health professionals to voice 
their evidence-based understandings of climate-sensitive pathologies 
to lobby for political action. Other important systemic interventions 
in health care include achieving universal health coverage (including 
financial risk protection); equitable access to quality essential health 
care services as well as safe, effective, and affordable medicines and 
vaccines; making health care and health systems net-zero carbon; add­
ing a climate-change lens to existing lines of research; improving data 
quality and enhancing, standardizing, and integrating data collection 
in LMICs; and anticipating and correcting disaster-related health care 
system failures, such as impacts to supply chains or loss of electric 
power resulting from extreme weather events.
PART 15
Disorders Associated with Environmental Exposures 
Interventions related to environmental policy include advocat­
ing for a tighter particulate-matter air-quality standard, supporting 
institutional divestment from fossil fuels, and advocating for the rapid 
drawdown of emissions and negative emissions strategies. Ecosocial 
interventions, supported by national or global institutions, include the 
distribution of clean cookstoves globally, switching to plant-based diets, 
decreased air travel, reducing air conditioner use, and increased access 
to more public transportation. Finally, advocating for wealth-redistribution 
schemes (e.g., reparations, progressive taxation, debt cancellation, 
improved safety nets, underemployment insurance) to empower dis­
advantaged populations to cope with climate hazards will have positive 
ancillary effects on the social determinants of health, the administration 
of health services, and the outcomes of clinical interventions.
CONCLUSIONS
Without sweeping reductions in GHG emissions, over the next 50 to 100 
years, models predict increases in average global temperature of 2–5°C 
(with localized highs), rising sea levels, and more frequent and severe 
extreme-weather events, with resultant complications for population 
health globally. The hostile consequences of climate change will dis­
proportionately affect vulnerable and marginalized groups, particularly 
those whose ability to cope with climate hazards is curtailed by systemic 
racism, colonial legacies, illicit financial flows, and human rights failings.
Health care professionals find themselves on the front line of the 
climate crisis and remain, in many settings, sources of information and 
counsel. In order to mitigate the impact of climate-sensitive diseases and 
resulting health disparities, they must continue to extend their clinical 
purview to socioecological determinants and structural interventions.

Reduced risk of cardiovascular
disease, T2D, stroke,
cancer, obesity
Increased biking/walking
Impact on nutrition
Improved food security
Preservation of protein and
micronutrient content of crops
Acknowledgment
Paul Farmer contributed to this chapter in the 21st edition and some 
material from that chapter has been retained here.
■
■FURTHER READING
Bekkar B et al: Association of air pollution and heat exposure with 
preterm birth, low birth weight, and stillbirth in the US: A systematic 
review. JAMA Netw Open 3:e208243, 2020.
Brief of Amici Curiae Public Health Experts, Public Health Organiza­
tion, and Doctors In Support of Plaintiffs-Appellees’ Petition for 
Hearing En Banc, Juliana v. United States of America, No. 18-36082 
(9th Cir. Mar. 13, 2020).
Centers for Disease Control and Prevention: Climate Effects on 
Health. Available from https://www.cdc.gov/climateandhealth/effects/
default.htm.  Accessed January 19, 2024.
Hoffman JS et al: The effects of historical housing policies on resident 
exposure to intra-urban heat: A study of 108 US urban areas. Climate 
8:12, 2020.
The Lancet: Countdown on health and climate. Available at https://
www.thelancet.com/countdown-health-climate-  Accessed January 19, 
2024.
The New England Journal of Medicine: Climate Change. Available 
from https://www.nejm.org/climate-crisis  Accessed January 19, 2024.
Rajagopalan S et al: Air pollution and cardiovascular disease: JACC 
state-of-the-art review. J Am Coll Cardiol 72:2054, 2018.
Aaron S. Bernstein, Jonathan A. Patz

Climate Change and 

Infectious Disease
Since the late nineteenth century, humans have released greenhouse 
gases—mainly carbon dioxide and methane—into the atmosphere, 
creating a new climate unseen in human times. This new climate has 
already altered the epidemiology of infectious diseases, and the accu­
mulation of more greenhouse gases in the atmosphere will further alter 
the incidence and severity of infections. In certain instances, climate

change may establish conditions promoting the emergence of infec­
tious diseases, while in other instances, it may render areas that are 
presently suitable for certain diseases unsuitable. This chapter presents 
the current state of knowledge regarding the known and prospective 
infectious-disease consequences of climate change.
OVERVIEW
The term climate change refers to multidecadal alterations in temperature, 
precipitation, wind, humidity, and other components of weather outside 
of the natural climate variability seen in comparable time periods. Over 
the past 2.5 million years, the earth has warmed and cooled, cycling 
between glacial and interglacial periods during which average global tem­
peratures moved up and down by 4–7°C. During the last glacial period, 
which ended roughly 12,000 years ago, global temperatures were, on aver­
age, 5°C cooler than in the mid-twentieth century (Fig. 474-1).
The present climate period, known as the Holocene, is remarkable 
for its stability: temperatures have largely remained within a range of 
2–3°C. This stability has enabled the successful population and culti­
vation of much of the earth’s landmass by humanity. Current climate 
change differs from that in the past not only because its primary cause 
is human activities but also because its pace is faster. The current rate 
Observed warming
Contributions to warming based on two complementary approaches
(a) Observed warming
2010–2019 relative 
to 1850–1900
(b) Aggregated contributions to 
(c) Contributions to 2010–2019
 
2010–2019 warming relative  
 
warming relative to 1850–1990,
 
to 1850–1990, assessed from 
 
assessed from radiative
 
attribution studies 
 
forcing studies
°C
2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
Internal variability
Solar and volcanic drivers
Other human drivers
Well-mixed greenhouse gases
Total human influence
FIGURE 474-1  Assessed contributions to observed warming in 2010–2019 relative to 1850–1900. Assessed contributions to observed warming in 2010–2019 relative to 1850–
1900. A. Observed global warming (increase in global surface temperature). Whiskers show the very likely range. B. Evidence from attribution studies, which synthesize 
information from climate models and observations. Temperature change attributed to total human influence; changes in well-mixed greenhouse gas (GHG) concentrations; 
other human drivers due to aerosols, ozone, and land-use change (land-use reflectance); solar and volcanic drivers; and internal climate variability. Whiskers show likely 
ranges. C. Evidence from the assessment of radiative forcing and climate sensitivity. Temperature changes from individual components of human influence: emissions of 
GHGs, aerosols, and their precursors; land-use changes (land-use reflectance and irrigation); and aviation contrails. Whiskers show very likely ranges. Estimates account 
for both direct emissions into the atmosphere and their effect, if any, on other climate drivers. For aerosols, both direct effects (through radiation) and indirect effects 
(through interactions with clouds) are considered. (Used with permission from IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. 
Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [V Masson-Delmotte et al (eds.)]. Cambridge University 
Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3−32, doi:10.1017/9781009157896.001. Reproduced with permission of the Licensor through PLSclear.)

of warming on Earth is unprecedented in the last 50 million years. 
The 5°C of warming that occurred at the end of the last ice age about 
12,000 years ago took roughly 5000 years, whereas such a temperature 
increment may occur within the next 150 years unless the release of 
greenhouse gases is substantially reduced in coming decades.

■
■GREENHOUSE GASES
Greenhouse gases (Table 474-1) are a group of gases in Earth’s atmo­
sphere that absorb infrared radiation and thus warm the planet. Essen­
tially, greenhouse gases act as a blanket on the earth to keep more of 
the sun’s solar radiation in the atmosphere. Carbon dioxide, released 
into the atmosphere primarily from fossil fuel combustion and defor­
estation, has had the greatest effect on climate since the Industrial 
Revolution. Other greenhouse gases, such as methane, nitrous oxides, 
and fluorinated gases, are more potent than carbon dioxide, but make 
up a smaller proportion of greenhouse gases. Both carbon dioxide and 
methane have increased considerably from the nineteenth century 
until 2023, with methane concentrations more than doubling. Water 
vapor is the most abundant and a highly potent greenhouse gas but, 
given its short atmospheric life span and sensitivity to temperature, is 
not a major factor in recently observed warming.
°C
2.0
°C
2.0
CHAPTER 474
1.5
1.5
1.0
1.0
Climate Change and Infectious Disease 
0.5
0.5
0.0
0.0
–0.5
–0.5
–1.0
–1.0
Aviation contrails
Land-use reflectance
and irrigation
Black carbon
Ammonia
Organic carbon
Sulphur dioxide
Volatile organic compounds
and carbon monoxide
Nitrogen oxides
Halogenated gases
Nitrous oxide
Methane
Carbon dioxide
Mainly contribute
to changes in
anthropogenic
aerosols
Mainly contribute
to changes in
non-CO2 greenhouse
gases

TABLE 474-1  Greenhouse Gases: Sources, Sinks, and Forcings
GAS
HUMAN SOURCES
SINKa
RADIATIVE FORCINGb (95% 
CONFIDENCE INTERVAL)
Carbon dioxide (CO2)
Fossil fuel combustion, deforestation
Uptake by oceans (~30%), plants
1.68 (1.33–2.03)
Methane (CH4)
Fossil fuel production, ruminant animals, decomposition in landfills
Hydroxyl radicals in the troposphere
0.97 (0.74–1.20)
Nitrous oxide (N2O)
Fertilizer, fossil fuel combustion, biomass burning, 

livestock manure
Halocarbons
Refrigerants, electrical insulation, aluminum production
Hydroxyl radicals in the troposphere, 
sunlight in the stratosphere
aIn this table, a sink refers to the place where greenhouse gases are naturally stored or the mechanism through which they are destroyed. bRadiative forcing, measured in 
watts per meter squared, refers to how much an entity can alter the balance of incoming and outgoing radiation to and from Earth’s atmosphere. It is measured relative to a 
preindustrial (i.e., 1750) baseline. Greenhouse gases have a positive “forcing”; that is, on balance, they increase the amount of radiation (and specifically infrared radiation) 
that is retained in Earth’s atmosphere.
Source: Intergovernmental Panel on Climate Change Fifth Assessment Report, Working Group 1, Chapter 8; American Chemical Society “Greenhouse gas sources and 
sinks,” available at www.acs.org/content/acs/en/climatescience/greenhousegases/sourcesandsinks.html.
The atmosphere, some of the aerosols suspended in it, and clouds 
reflect a portion of incoming solar radiation back toward space. The 
remainder reaches Earth’s surface, where it is absorbed and some is 
emitted back into the atmosphere. The earth emits energy absorbed 
from the sun at longer wavelengths, primarily infrared, that green­
house gases can absorb. The change in wavelength that occurs as solar 
radiation is absorbed and re-emitted from the earth’s surface is funda­
mental to the greenhouse effect (Fig. 474-2).
■
■TEMPERATURE
Climate change has clearly caused global warming with the average 
surface temperature of Earth increasing 1.09°C from 1880 to 2020. 
Moreover, the rate of global warming is faster now than at any time in 
the last 1000 years. Yet, this mean warming fails to show that warming 
is occurring much faster in certain regions. The Arctic has warmed 
twice as fast overall, and winters are warming faster than summers. 
Nighttime minimum temperatures are also rising faster than daytime 
high temperatures. Each of these nuances bears upon the incidence 
of infectious diseases in general and vector-borne disease specifically.
PART 15
Disorders Associated with Environmental Exposures 
Due to climate change, extreme heat waves are expected to be more 
common, longer, and more severe in the future. The hottest 5-year 
Incoming solar
radiation
342 W m–2
Reflected solar
radiation
107 W m–2

Reflected by
clouds, aerosol
and atmosphere

Reflected by
the surface

Absorbed by
the surface
Thermals

FIGURE 474-2  Earth’s energy balance. (Kiehl’s Earth’s Annual Global Mean Energy Budget, Bulletin of the American Meteorological Society, Vol. 78, No. 2, 1997 (Figure 7, 
page 206). © American Meteorological Society. Used with permission.)

Photolysis in the stratosphere
0.17 (0.14–0.23)
0.18 (0.01–0.35)
period ever recorded since records started in the mid-nineteenth cen­
tury was 2016–2020. Besides contributing directly to morbidity and 
mortality in human populations, heat waves wilt crops and are pre­
dicted to contribute substantially to agricultural losses. For example, 
the 2010 heat wave in Russia, which was unprecedented in its severity, 
contributed to hundreds of forest fires that generated enough air pol­
lution to kill an estimated 56,000 people and that burned 300,000 acres 
of crops, including roughly 25% of the nation’s wheat fields. Nutritional 
deficiencies underlie a substantial portion of the global burden of many 
infectious diseases.
■
■PRECIPITATION
In addition to changing temperature, the emission of greenhouse gases and 
the consequent increase in energy in Earth’s atmosphere have influenced 
the planet’s water cycle. Since 1950, substantial increases in the heavi­
est precipitation events (i.e., those above the 95th percentile) have been 
observed in Europe and North America. Moreover, in 2022, floods harmed 
an estimated 58 million people globally. Other areas have seen greater 
drought, notably southern Australia and the southwestern United States.
A warmer atmosphere holds more water vapor. Specifically, air holds 
6–7.5% more water vapor per degree (Celsius) of warming in the lower 
Outgoing longwave
radiation
235 W m–2

Emitted by the
atmosphere
Atmospheric
window

Absorbed by the
atmosphere

Greenhouse
gases
Latent
heat

Back
radiation

Surface
radiation
Evapotranspiration

Absorbed by
the surface

atmosphere. For areas that have traditionally had more precipitation 
on average, warming tends to promote heavier precipitation events. 
In contrast, in regions prone to drought, warming tends to result in 
greater periods between rainfalls and in the risk of drought. Floods and 
droughts have been associated with outbreaks of waterborne infectious 
diseases.
■
■HURRICANES
The world’s oceans have absorbed 90% of the excess heat that green­
house gases have kept in Earth’s atmosphere since the 1960s. Ocean 
heat provides energy for hurricanes, and warmer years tend to have 
greater hurricane activity. Stronger hurricanes (category 4 and 5) are 
expected with climate change, though climate change influence on 
hurricane frequency is uncertain. Modeling of future tropical cyclones 
suggests that their intensity may increase 2–11% by 2100 and that the 
average storm will bring 20% more rainfall.
■
■SEA LEVEL RISE
Between 1901 and 2010, the global sea level rose ~200 mm, or ~1.7 mm 
per year on average. From 1993 to 2010, the rate of rise nearly dou­
bled—i.e., to 3.2 mm annually. Most of this sea level rise has resulted 
from the thermal expansion of water. Glacial ice melt is the second 
greatest factor, and its contribution is 
accelerating. By 2100, global sea level 
may rise by 0.8–2 m, with an annual rate 
of rise of 8–16 mm at the century’s end.
Sea level rise is not uniform. The rate 
of rise on the eastern seaboard of North 
America has been roughly double the 
global rate. Compounding sea level rise 
is the subsidence of coastal areas due 
to human settlement. In the absence of 
levee upgrades, an estimated 300 million 
people living near coasts worldwide will 
be at risk of flooding in 2050 because of 
the combined effects of subsidence, ero­
sion, and sea level rise.
Along with extreme storms and 
overuse of coastal aquifers, rising seas 
also contribute to salinization of coastal 
groundwater. About 1 billion people rely 
on coastal aquifers for potable water.
■
■EL NIÑO SOUTHERN 
OSCILLATION
The El Niño Southern Oscillation (ENSO) 
refers to periodic changes in water tem­
perature in the eastern Pacific Ocean that 
occur roughly every 4–5 years. ENSO 
cycles have dramatic effects on weather 
around the globe. Warmer-than-aver­
age water temperatures in the eastern 
Pacific define El Niño events (see below), 
whereas cooler-than-average water tem­
peratures define La Niña periods. Evi­
dence is accruing that climate change 
may be increasing the frequency and 
severity of El Niño events.
El Niño events drive alterations in 
weather worldwide (Fig. 474-3) and are 
associated with extreme events and con­
sequently higher rates of morbidity and 
mortality. Hurricane Mitch, one of the 
most powerful hurricanes ever observed, 
with winds reaching 290 km/h, dropped 
1–1.8 m (3–6 feet) of rain over 72 h on 
parts of Honduras and Nicaragua. As 
a result of this storm, 11,000 people 
died and 2.7 million were displaced. 
FIGURE 474-3  Characteristic weather anomalies, by season, during El Niño events. (Source: Climate Prediction 
Center, https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/impacts/warm_impacts.shtml.)

Outbreaks of cholera, leptospirosis, and dengue occurred in the storm’s 
aftermath.

■
■POPULATION MIGRATION AND CONFLICT
The final common outcome of all climate-change effects is human dis­
placement. Sea level rise, extreme heat and precipitation, droughts, and 
salinization of water supplies all conspire to make regions, including 
some inhabited by humans for millennia, uninhabitable. The 8 million 
inhabitants of low-lying South Pacific islands are vulnerable to sea level 
rise and could be a major source of climate migrants.
Climate change may also be contributing to humanitarian crises 
and conflicts. A severe 2011 drought in East Africa may have incited 
the Somali famine that resulted in 1 million refugees; mortality rates 
reached 7.4/10,000 in some refugee camps.
EFFECTS OF CLIMATE CHANGE ON 
INFECTIOUS DISEASE
The incidence of most, if not all, infectious diseases depends on cli­
mate. For any given infection, however, climate change is but one of 
many factors determining disease epidemiology. In instances in which 
climate change creates conditions favorable to the spread of infections, 
CHAPTER 474
Climate Change and Infectious Disease

diseases may be kept in check through interventions such as vector 
control or antibiotic treatment.

Detecting the influence of climate change amid the many compet­
ing forces that bear on infectious disease emergence and spread can be 
challenging. Research on animal pathogens, which in most instances 
are less intervened upon than that with their human counterparts, has 
suggested how climate change may independently influence disease 
spread. Avian malaria in Hawaii, for example, has clearly moved to 
higher elevations where warmer temperatures have enabled disease 
transmission. Many putative pathways have been identified that con­
nect greenhouse gas emissions to infectious disease risk in people. Cli­
mate change influences extend beyond contagious diseases and include 
diseases from microbial toxins, such as those that result from harmful 
algal blooms. Warmer ocean temperatures cause Pseudonitzschia spp., 
blue–green cyanobacteria, and dinoflagellates to grow faster, resulting 
in concurrent disease outbreaks.
■
■VECTOR-BORNE DISEASE
Because insects are cold-blooded, ambient temperature dictates their 
geographic distribution. With increases in temperatures (in particular, 
nighttime minimum temperatures), insects are freed to move pole­
ward and up mountainsides. At the same time, as new areas become 
climatically suitable, current mosquito habitats may become unsuitable 
because of heat extremes.
In addition, insects tend to be sensitive to water availability. Mos­
quitoes that transmit malaria, dengue, and other infections may breed 
in pools of water created by heavy downpours. As has been observed 
in the Amazon, breeding pools can also appear during periods of 
drought when rivers recede and leave behind stagnant pools of water 
for Anopheles mosquitoes. These circumstances have raised interest in 
the potentially favorable impact of water-cycle intensification on the 
spread of mosquito-borne disease.
PART 15
Disorders Associated with Environmental Exposures 
Malaria 
• 
TEMPERATURE  Higher temperatures promote higher 
mosquito-biting rates, shorter parasite reproductive cycles, and the 
potential for the survival of mosquito vectors of Plasmodium infection 
in locations previously too cold to sustain them. Modeling experiments 
have identified highland areas of East Africa and South America as 
perhaps most vulnerable to increased malarial incidence as a result of 
rising temperatures. In addition, an analysis of interannual malaria in 
Ecuador and Colombia has documented a greater incidence of malaria 
at higher altitudes in warmer years. Highland populations may be more 
vulnerable to malaria epidemics because they lack immunity.
Although rising temperature has the potential to expand the viable 
range of disease, malaria incidence is not linearly associated with 
temperature. While mosquitoes and parasites may adapt to a warm­
ing climate, the present optimal temperature for malaria transmission 
is ~25°C, with a range of transmission temperatures between 16°C 
and 34°C. Rising temperatures can also have differential effects on 
parasite development during external incubation and on the mosqui­
toes’ gonotrophic cycle. Asynchrony between these two temperaturesensitive processes has been shown to decrease the vectorial capacity 
of mosquitoes.1
PRECIPITATION  The abundance of Anopheles mosquitoes is strongly 
correlated with the availability of surface-water pools for mosquito 
breeding, and biting rates have been linked to soil moisture (a surrogate 
for breeding pools). Research in the East African highlands has docu­
mented that increased variance in rainfall over time has strengthened 
the association between precipitation and disease incidence. These 
disease-promoting effects of precipitation may be countered by the 
potential for extreme rainfall to flush mosquito larvae from breeding 
sites.
1rVc is the vectorial capacity relative to the vector-to-human population ratio 
and is defined by the equation rVc = a2bhbme–μmn/μm where a is the vector bit­
ing rate; bh is the probability of vector-to-human transmission per bite; bm is 
the probability of human-to-vector infection per bite; n is the duration of the 
extrinsic incubation period; and μm is the vector mortality rate.

PROJECTIONS  Climate models have begun to deliver output on 
regional scales, permitting projections of climate-suitable regions to 
assist national and local health authorities. Climate models speak to the 
temperature and precipitation ranges necessary for malaria transmis­
sion but do not account for the capacity of malaria control programs to 
halt the spread of disease. The global reduction in malaria distribution 
over the past century makes it clear that, even with climate change, 
malaria occurs in far fewer places today because of public health inter­
ventions. Recent vaccine trials also show promise in further reducing 
malaria risk.
Despite intensive efforts, malaria remains the single greatest vectorborne disease cause of morbidity and death in the world. Particularly 
in regions that are most affected by malaria and where the public health 
infrastructure is inadequate to contain it, climate modeling may pro­
vide a useful tool in determining where the disease may spread. Mod­
eling studies in sub-Saharan Africa have suggested that, although East 
African nations may encompass regions that will become more climati­
cally suitable for malaria over this century, West African nations may 
not. By 2100, temperatures in West Africa may largely exceed those 
optimal for malaria transmission, and the climate may become drier; in 
contrast, higher temperatures and changes in precipitation may allow 
malaria to move up the mountainsides of East African countries. Cli­
mate change may create conditions favorable to malaria in subtropical 
and temperate regions of the Americas, Europe, and Asia as well.
Dengue 
Like malaria epidemics, dengue fever epidemics depend 
on temperature (Fig. 474-4). Higher temperatures increase the rate of 
larval development and accelerate the emergence of adult Aedes mos­
quitoes. The daily temperature range may also influence dengue virus 
transmission, with a smaller range corresponding to a higher trans­
mission potential. Temperatures <15°C or >36°C substantially reduce 
mosquito feeding. In a Rhesus model of dengue, viral replication can 
occur in as little as 7 days with temperatures of >32–35°C; at 30°C, rep­
lication takes ≥12 days; and replication does not reliably occur at 26°C. 
Research on dengue in New Caledonia has shown peak transmission 
at ~32°C, reflecting combined effects of a shorter extrinsic incubation 
period, a higher feeding frequency, and more rapid development of 
mosquitoes. Along with temperature, peak relative humidity is a strong 
predictor of dengue outbreaks.
The association between dengue epidemics and precipitation is less 
consistent in the peer-reviewed literature, possibly because of the mos­
quito vector’s greater reliance on domestic breeding sites than on natu­
ral pools of water. For instance, in some studies, increased access to a 
piped water supply has been linked to dengue epidemics, presumably 
because of associated increased domestic water storage. Nonetheless, 
several studies have established rainfall as a predictor of the seasonal 
timing of dengue epidemics.
The current global distribution of dengue largely overlaps the 
geographic spread of Aedes mosquitoes (Fig. 474-5). The presence of 
Aedes without dengue endemicity in large regions of North and South 
America and Africa illustrates the relevance of variables other than climate 
to disease incidence. Nevertheless, coupled climatic–epidemiologic 
modeling suggests dramatic shifts in the relative vectorial capacity 
for dengue by the end of this century should little or no mitigation of 
greenhouse gas emissions occur (Fig. 474-6).
Other Arbovirus Infections 
Climate change may favor increased 
geographic spread of other arboviral diseases, including Zika virus 
disease, chikungunya virus disease, West Nile virus disease, and east­
ern equine encephalitis. Zika virus moved to the Western Hemisphere 
from French Polynesia around 2013 and rapidly spread in Brazil in 
2016. Although air travel was essential for the delivery of the virus to 
the Americas, the available evidence suggests that the 2015 El Niño 
event provided an optimal climate for the infection to take root and 
spread (see “ENSO-Related Outbreaks,” below). Aedes aegypti is the 
primary vector for Zika virus. Chikungunya virus disease emerged 
in Italy in 2007, having previously been mostly a disease of African 
nations. Climate models predict that, should competent vectors be 
present, conditions will be suitable for the chikungunya virus to gain 
a foothold in Western Europe, especially France, in the first half of

Days (EIP length and development to adult)

Temperature (°C)

FIGURE 474-4  Effects of temperature on variables associated with dengue transmission. Shown are the 
number of days required for development of immature Aedes aegypti mosquitoes to adults, the length of the 
dengue virus type 2 extrinsic incubation period (EIP), the percentage of Ae. aegypti mosquitoes that complete 
a blood meal within 30 min after a blood source is made available, and the percentage of hatched Ae. aegypti 
larvae surviving to adulthood. (Reproduced from CW Morin et al: Climate and dengue transmission: Evidence 
and implications. Environ Health Perspect 121:1264, 2013.)
the twenty-first century. In North America, areas favorable to West 
Nile virus outbreaks are expected to shift northward in this century. 
Current hotspots in North America are the California Central Valley, 
southwestern Arizona, southern Texas, and Louisiana, which have 
both compatible climates and avian reservoirs for the disease. By midcentury, the upper Midwest and New England will be more climatically 
suited to West Nile virus; by the end of the century, the area of risk may 
shift even further north to southern Canada. Whether the disease will 
ultimately move northward will depend on reservoir availability and 
mosquito control programs, among other factors.
Lyme Disease 
In the past few decades, Ixodes scapularis, the 
primary tick vector for Lyme disease as well as for anaplasmosis and 
FIGURE 474-5  Distribution of Aedes aegypti mosquitoes (turquoise) and dengue fever epidemics (red). (Map produced by the Agricultural Research Service of the U.S. 
Department of Agriculture.)

babesiosis in New England, has become estab­
lished in Canada because of warming tempera­
tures. With climate change, the range of the tick is 
expected to expand further (Fig. 474-7).

Development to adult

EIP length
Percent (survival to adult and blood fed)
Blood fed

Survival to adult
Lyme disease, caused by the spirochete Bor­
relia burgdorferi, is the most commonly reported 
vector-borne disease in North America, with 
~60,000 cases annually. In Europe, Lyme disease 
has also increased and expanded geographically. 
Furthermore, the timing and peaks of cases have 
been affected, with the annual peak case numbers 
in 2019 arriving 6 weeks earlier than 25 years 
prior.

■
■WATERBORNE DISEASE
Many microorganisms, from bacteria to toxinproducing algae, cause waterborne disease (Table 
474-2). The outbreaks of waterborne disease are 
associated with heavy rainfall events. A review of 
548 waterborne disease outbreaks in the United 
States found that 51% were preceded by pre­
cipitation levels above the 90th percentile. Since 
1900, most regions of the United States except 
the Southwest and Hawaii have experienced an 
increase in heavy downpours with the greatest 
intensification of the water cycle in New England 
and Alaska.
Most disease outbreaks after heavy precipitation occur through 
contamination of drinking-water supplies. While outbreaks related to 
surface-water contamination generally occur within a month of the 
precipitation event, disease outbreaks from groundwater contamina­
tion tend to occur ≥2 months later. According to a review of published 
reports of waterborne disease outbreaks, Vibrio and Leptospira spe­
cies are the pathogens most commonly involved in the wake of heavy 
precipitation.

CHAPTER 474
Climate Change and Infectious Disease 
Combined Sewer Systems 
Roughly 40 million people in the 
United States and millions more around the world rely on combined 
sewer systems in which storm water and sanitary wastewater are 
conveyed in the same pipe to treatment facilities. These systems were

A
PART 15
Disorders Associated with Environmental Exposures 
B
FIGURE 474-6  Trend of annually averaged global dengue epidemic potential (rVc). Differences in rVc are based on 
30-year averages of temperature and daily temperature range. A. Differences between 1980–2009 and 1901–1930. 
B. Differences between 2070–2099 and 1980–2009. The mean value of rVc was averaged from five global climate 
models under RCP8.5, a scenario of high greenhouse-gas emission. The color bar describes the values of the rVc. 
(From J Liu-Helmersson et al: Vectorial capacity of Aedes aegypti: Effects of temperature and implications for global 
dengue epidemic potential. PLoS One 9:e89783, 2014.)
designed based on the nineteenth-century climate, in which heavy 
downpours were less frequent than they are today. The frequency of 
combined sewer overflows usually into freshwater bodies and resulting 
in untreated sewage and runoff potentially containing heavy metals 
and pesticides, has been increasing in cities worldwide. For instance, 
the channel leading into Lake Michigan from Milwaukee had its high­
est levels of Escherichia coli, up to 100 times the Environmental Protec­
tion Agency guidance for recreational waters, after combined sewer 
overflows (Fig. 474-8).
Rising Temperatures and Vibrio Species 
Warmer temperatures 
favor proliferation of Vibrio species and disease outbreaks, as has been 
demonstrated in countries surrounding the Baltic Sea, Chile, Israel, 
northwestern Spain, and the U.S. Pacific Northwest. Around the Baltic 
Sea, outbreaks of Vibrio infection may be particularly likely because of 
faster warming near the poles and the sea’s relatively low salt content. 
In the United States, levels of vibriosis roughly tripled from 1996–2010, 
with the highest number of cases occurring each summer.
ENSO-Related Outbreaks 
Weather extremes tied to El Niño 
events afford a window into a climate-changed future. Recent evidence 
indicates that climate change itself may be strengthening El Niño 
events. These events tend to promote epidemic infections in certain 
regions (Fig. 474-9).
Associations of El Niño with outbreaks of Rift Valley fever in east­
ern and southern Africa have been known since the 1950s. El Niño 
favors wet conditions suitable for the insect vectors of the disease 

in these regions. Given the strong asso­
ciation between El Niño conditions and 
disease incidence, models have successfully 
predicted Rift Valley fever epidemics in 
humans and animals. In the 2006–2007 El 
Niño season, for example, outbreaks of 
Rift Valley fever were accurately predicted 
2–6 weeks prior to epidemics in Somalia, 
Kenya, and Tanzania.
0.21
0.12
0.06
0.03
–0.03
El Niño has had inconsistent associations 
with malaria incidence in African countries. 
Some of the strongest associations between 
El Niño and malaria have been identified in 
South Africa and Swaziland, where available 
data on incidence are relatively robust; how­
ever, even in these instances, the observed 
increased risk did not reach statistical sig­
nificance. A stronger link to El Niño has 
been found in several studies done in South 
America. Research on malaria incidence in 
Colombia between 1960 and 2006 found 
that a 1°C temperature rise contributed to a 
20% increase in incidence.
–0.06
0.52
0.41
0.26
0.12
In the desert Southwest of the United 
States, hantavirus erupted following the 
strong El Niño of 1997. Unseasonal rain­
fall caused the desert to bloom, providing 
food resources and resulting in a boon 
to the mouse population. The following 
year saw the climate revert back to normal 
(arid) conditions, and as a result, increased 
human/mouse contact occurred as mice 
were forced to seek out food and nest in 
human dwellings.
0.06
0.03
–0.03
–0.06
–0.12
–0.26
–0.41
–0.52
In Bangladesh, a strong association 
has been observed between the El Niño 
Southern Oscillation Index and cholera epi­
demics. During El Niño, a combination 
of warmer sea-surface temperatures (SSTs) 
and rich nutrient runoff lead to phytoplank­
ton blooms detectable by satellite. These 
blooms, in turn, feed zooplankton, which 
can harbor V. cholerae and, therefore, amplify their presence in the 
environment, causing cholera epidemics in the region (Fig. 474-10).
El Niño years are often associated with an increased incidence of 
dengue. Research on dengue outbreaks in Thailand from 1996 to 2005 
revealed that 15–22% of the variance in monthly dengue disease inci­
dence was attributable to El Niño. In South America, data on dengue 
outbreaks between 1995 and 2010 showed an increased incidence dur­
ing the El Niño events of 1997–1998 and 2006–2007.
El Niño may have contributed to the emergence of Zika virus in the 
wake of a very strong ENSO event during the winter of 2015–2016. 
Temperatures throughout South and Central America were exception­
ally high and, just prior to the Zika epidemic, the vectorial capacity of 
Ae. aegypti was at its highest level compared with the previous 60 years. 
Not surprisingly (because carried by the same mosquito vector), large 
epidemics of dengue also occurred in Brazil and Colombia at that time. 
But laboratory studies show that the temperature optimum for Zika 
virus is 5°C warmer than for dengue. Thus, it is possible the extreme 
temperature played a role in the extent of the Zika epidemic.
CLIMATE CHANGE, POPULATION 
DISPLACEMENT, AND INFECTIOUS 
DISEASE EPIDEMICS
For many reasons, including freshwater shortages, flooding, food 
shortages, and climate change–driven conflicts, climate change has and 
will continue to put pressure on human populations to move. Human 
migrations have long been associated with epidemic disease in the

2000–2019
2000–2019
N
A
2020–2049
2020–2049
N
B
F
2050–2079
2050–2079
N
C
G
2080+
2080+
N
D
H
= High risk
= Moderate risk
= Low risk
= Risk of bird-borne ‘adventitious’ ticks only
FIGURE 474-7  Risk maps for the occurrence of the Lyme disease vector Ixodes scapularis. Expansion of I. scapularis–affected census subdivisions (CSDs) in Canada from 
the present (using 1971–2000 temperature normals) to the 2080s (using temperature conditions predicted by the CGCM2 climate model under emissions scenario A2). In A–D 
(“slow” scenario), the model assumes that by the end of each time period, only risk CSDs with an algorithm value in the top 10% will contain an I. scapularis population. 
In E–H (“fast” scenario), the model assumes that by the end of each time period, all CSDs within the “moderate”-risk zone for I. scapularis establishment (risk CSDs) 
contain an I. scapularis population. For both scenarios, the time steps are 2000–2019, 2020–2049, 2050–2079, and 2080– 2109. “High”-risk regions for I. scapularis population 
establishment are indicated in red. “Moderate”-risk regions are in orange. “Low”-risk regions are in yellow. Regions with no risk of established populations but some risk 
from bird-borne “adventitious” ticks are in green. Regions with no predicted risk of either are colorless. (Used from NH Ogden: Risk maps for range expansion of the Lyme 
disease vector, Ixodes scapularis, in Canada now and with climate change. Int J Health Geogr 7:1, 2008.)

N
E
N
CHAPTER 474
N
Climate Change and Infectious Disease 
N

TABLE 474-2  Climate Sensitive Agents of Water-Related Illness
PATHOGEN OR TOXIN PRODUCER
EXPOSURE PATHWAY
SELECTED HEALTH OUTCOMES AND SYMPTOMS
Algae: Toxigenic marine species 
of Alexandrium, Pseudo-nitzschia, 
Dinophysis, Gambierdiscus; Karenia brevis
Shellfish, fish
Recreational waters 
(aerosolized toxins)
Gastrointestinal and neurologic illness caused by 
shellfish poisoning (paralytic, amnesic, diarrhetic, 
neurotoxic) or fish poisoning (ciguatera). Asthma 
exacerbations, eye irritations caused by contact with 
aerosolized toxins (K. brevis).
Cyanobacteria (multiple freshwater 
species producing toxins including 
microcystin)
Drinking water
Recreational waters
Liver and kidney damage, gastroenteritis (diarrhea and 
vomiting), neurologic disorders, and respiratory arrest.
Enteric bacteria and protozoan parasites: 
Salmonella enterica; Campylobacter 
species; toxigenic Escherichia coli; 
Cryptosporidium; Giardia
Drinking water
Recreational waters
Shellfish
Enteric pathogens generally cause gastroenteritis. 
Some cases may be severe and may be associated 
with long-term and recurring effects.
Enteric viruses: enteroviruses; rotaviruses; 
noroviruses; hepatitis A and E
Drinking water
Recreational waters
Shellfish
Most cases result in gastrointestinal illness. Severe 
outcomes may include paralysis and infection of the 
heart or other organs.
Leptospira and Leptonema bacteria
Recreational waters
Mild to severe flu-like illness (with or without fever) to 
severe cases of meningitis, kidney, and liver failure.
Vibrio bacteria species
Recreational waters
Shellfish
Varies by species but include gastroenteritis 
(V. parahaemolyticus, V. cholerae), septicemia 
(bloodstream infection) through ingestion or wounds 
(V. vulnificus), skin, eye, and ear infections 

(V. alginolyticus).
Source: Climate impacts on water-related illness, in The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. U.S. Global Change 
Research Program, Washington, DC, 2016. https://health2016.globalchange.gov/water-related-illness.
migrating populations themselves and in the communities in which 
they settle. The specific pathogens and patterns of disease that may 
appear after population migration relate to endemic diseases present 
in the migrant populations.
PART 15
Disorders Associated with Environmental Exposures 
Large-scale migrations are common after extreme precipitation 
events. In August 2022, Pakistan experienced over three times the 
average rainfall, resulting in extensive flooding. Roughly 33 million 
people were affected, over 1400 people died, and 1.7 million homes 
were destroyed, all resulting in large-scale displacement. Additionally, 
standing water provided ideal breeding sites for vectors, and contami­
nated flood waters in two southern provinces led to outbreaks of diar­
rhea, cholera, and malaria. Accelerated computing power now allows 
1e+5
1e+4
CFU/100 ml
1e+3
1e+2
1e+1
1e+0
CSO
Rain with
no CSO
Base flow
FIGURE 474-8  Levels of Escherichia coli in the Milwaukee estuary, which 
discharges to Lake Michigan, 2001–2007. Levels of E. coli in the Milwaukee estuary, 
which discharges to Lake Michigan, 2001–2007, during base flow (n = 46); following 
rain events with no CSO (n = 70); and following CSO events (n = 54). Boxes indicate 
75% of values, with median values drawn in each. Whiskers are 95% of values, and 
outliers are shown as closed circles. There were significant differences in E. coli 
levels following rainfall and CSOs compared to base flow (p ≤0.05). CFU, colony 
forming units; CSO, combined sewer overflow. (Reproduced with permission from 
JA Patz et al: Climate change and waterborne disease risk in the Great Lakes region 
of the U.S. Am J Prev Med 35:451, 2008.)

MAJOR CLIMATE CORRELATION 

OR DRIVER (STRONGEST DRIVERS 
LISTED FIRST)
Temperature (increased water 
temperature), ocean surface currents, 
ocean acidification, hurricanes 
(Gambierdiscus spp. and K. brevis)
Temperature, precipitation patterns
Temperature (air and water; both 
increase and decrease), heavy 
precipitation, and flooding
Heavy precipitation, flooding, and 
temperature (air and water; both 
increase and decrease)
Flooding, temperature (increased water 
temperature), heavy precipitation
Temperature (increased water 
temperature), sea level rise, 
precipitation patterns (as it affects 
coastal salinity)
for improved attribution of single weather events to climate change. 
For Pakistan, climate attribution analysis suggests that intense rainfall 
over short periods (5 days) has become more extreme with warming 
temperatures.
In low- and middle-income countries, infectious disease outbreaks 
associated with population displacement may be harder to detect and 
respond to. People forced to leave their homes en masse are at risk for 
contracting infections with any pathogen that may be present within 
the displaced population, including sexually transmitted diseases such 
as HIV, or airborne or droplet transmitted diseases such as tuberculosis 
and measles. Reducing disease risk requires overlaying of climaterelated migration risk with foci of disease epidemics.
A BROADER VIEW OF CLIMATE 

CHANGE AND HEALTH
While climate change has far-reaching implications for the distribu­
tion and severity of infectious diseases worldwide, the greatest disease 
burdens related to climate change may not be due to infections, at least 
primarily. Climate change erodes the foundations of health, such as 
safe drinking water and food security, due to climatic extremes such 
as flooding or droughts. In addition, resource scarcity and climate 
instability are increasingly associated with conflicts. For example, 
severe droughts made more likely by climate change may have been a 
factor in the revolutions of the Arab Spring and the Syrian civil war. Of 
course, without adequate nutrition, water, or shelter, infectious disease 
risks rise.
The public health response to climate change entails both mitiga­
tion and adaptation measures. Mitigation represents primary preven­
tion and involves the reduction of greenhouse gas emissions into the 
atmosphere. Although no clear safety threshold of greenhouse gas 
emissions has been agreed upon, in 2015 at the 21st Conference of 
the Parties (COP21) of the United Nations Framework Convention 
on Climate Change (UNFCCC) held in Paris, national governments 
from the major industrialized countries agreed to set a warming target 
of <2°C above preindustrial levels by 2050; the attainment of this goal 
will require reducing greenhouse gas emissions by 40–70% below 2010 
levels. At COP21, a framework was established for a global carbon mar­
ket and nationally determined contributions (NDCs) for countries to 
commit to greenhouse gas reductions to meet the Paris Climate Agree­
ment. At the 2023 Conference of the Parties (COP28) held in Dubai,

HPS, PL
MAL
CHOL
RVF
DENG
RI
MAL
FIGURE 474-9  Characteristic patterns of disease outbreaks associated with El Niño events, determined on the basis of 2006–2007 conditions. (From A Anyamba et al: 
Developing global climate anomalies suggest potential disease risks for 2006–2007. Int J Health Geogr 5:60, 2006.)
for the first time the “need to transition from fossil fuels” was explicitly 
included in the final text of the agreement.
Mitigation also confers health co-benefits, including better air qual­
ity and lower incidence and severity of respiratory infections, associ­
ated with less bio- or fossil fuel combustion. One study estimated that 
by eliminating air quality pollutants (PM 2.5, sulfur dioxide, nitrous 
oxides) from energy generation across the United States, >53,000 
premature deaths would be avoided. Of note, evidence has shown 
that long-term air pollution exposure may contribute to mortality 
risk from COVID-19 and influenza. Dietary and agricultural changes 
can also afford climate change mitigation and improve human health. 

Relevant Units
Jan 98
Jan 99
Jan 2000
Jan 2001
Jan 2002

Relevant Units
Jan 98
Jan 99
Jan 2000
Jan 2001
Jan 2002
FIGURE 474-10  Predicting cholera rates. Sea surface temperature, sea surface height, and chlorophyll-a predicting cholera in Bangladesh. Top graph illustrates 
environmental data detected by satellite measurements from the Bay of Bengal from January 1998–January 2002 and includes sea surface temperature (green line), sea 
surface height (blue line), and chlorophyll-a levels (yellow line), an indicator for the abundance of phytoplankton. Bottom graph illustrates the incidence rate of cholera 
(red) in Bangladesh over that time period. The black line combines the three environmental parameters and, superimposed over cholera incidence rate, shows a strong 
correlation. (Reproduced with permission from D Grimes et al: Viewing marine bacteria, their activity and response to environmental drivers from orbit: Satellite remote 
sensing of bacteria. Microb Ecol 67:489, 2014.)

DENG–Dengue Fever
RI–Respiratory Illness
CHOL–Cholera
MAL–Malaria
RVF–Rift Valley Fever
HPS–Hantavirus Pulmonary Syndrome
PL–Plague
CHOL
DENG
RI
Resource-intensive foods, like red meat and dairy, can lead to higher 
cases of diabetes and cardiovascular disease, so switching to plantbased diets can improve human health and reduce greenhouse gas 
emissions.
CHAPTER 474
Climate adaptation is secondary prevention and seeks to reduce 
harms associated with sea level rise, heat waves, floods, droughts, 
wildfires, and other greenhouse gas–driven events. The efficacy of 
adaptation is constrained by the challenges inherent in predicting 
the precise location, duration, and severity of extreme weather events 
and flooding related to sea level rise, among other considerations 
(Fig. 474-11).
Climate Change and Infectious Disease 
Sea Surface Temperatures
Sea Surface Heights
Chlorophyll-a Levels
Cholera Incident Rate
Predicted Cholera Rate