Emerging Infectious Diseases (EIDs) are illnesses that are newly defined or have existed but are increasing in incidence or geographic range which poses a threat to the population, either in a particular area or globally (1–3). Conversely, Re-Emerging Infectious Diseases (REIDs) are illnesses that existed in the past but reappear after they have been on a significant decline, and rapidly spread either in terms of incidence or to new geographical areas (2–5).

Many factors such as the destruction of natural habitats, which leads humans and animals to live in close proximity, changing climate and ecosystems, replacing reservoir hosts or intermediate insect vectors, and microbial genetic mutations are involved in the emergence of new diseases (6). Ecological changes due to natural disasters such as flood/drought and earthquakes, population migration resulting from war or conflict, interruption or decrease in disease prevention programs, and insufficient vector control applications and sanitation have great importance in disease emergence (1). Environmental and social determinants resulting from natural disasters, wars, etc. are also of great importance in disease re-emergences as well as viral and microbial factors (4). Major contributing factors to the prevalence of EIDs and REIDs are summarized in Table 1.

Re-emergence of diseases that were considered to have ended at the beginning of the 20^th century has begun in recent years (8). Wars and natural disasters have a great impact on the emergence/re-emergence of diseases in the population.

According to a World Bank estimation, two billion people are living in poverty and fragility situations (9). Wars destroy the health systems and infrastructure, curtail the existing disease control programs and cause population movement. Large population displacement into overcrowded camps and temporary shelters leads to an increase in exposure to health risks such as disease vectors favoring the emergence/re-emergence of infectious diseases (10, 11). Outbreaks of typhoid fever, cholera, dysentery, malaria, small-pox, typhus fever and influenza caused a significant number of deaths among soldiers and civilians in war zones during or after World War I and World War II (12, 13).

After a dramatic increase in morbidity/mortality rates in war-involved countries during and after World War I, tuberculosis was recognized as a war disease and became one of the main concerns associated with wars and conflicts (12, 14, 15). The disease re-emerged in Europe as a result of World War I where the incidence was declining (14). As most of current conflicts are prevalent in tuberculosis-endemic countries, tuberculosis continues to be an important threat not only to the people living in conflict areas, but also to the population living in unaffected areas and neighboring countries since huge numbers of people move to safer places internally or internationally (14–17).

One of the examples is the Syrian crisis which led millions of people to migrate to neighboring countries such as Türkiye. The proportion of foreign-country-born tuberculosis cases among total reported cases in Türkiye increased progressively from 1.1% in 2010 to 6.0% in 2014, 7.3% in 2017, and 10.84% in 2018. 587 Syrian cases constituted 53.0% of foreign-country-born cases and 4.87% of the total cases diagnosed in Türkiye in 2017 (18, 19).

The possibility of death in tuberculosis patients who received irregular or no treatment during the conflict was up to three times higher than the ones who received a full course of treatment during peacetime in Guinea-Bissau, West Africa (20, 21).

In a study assessing the impact of war on Ebola transmission and control in the Democratic Republic of the Congo (DRC), it was found that an increase in the number of cases had been observed over and over due to conflict conditions. Violence against healthcare workers and Ebola treatment centers inhibited the rapidity of case isolation, treatment, following up the contacts of cases, and vaccination programs due to continuous conflict events (22). War in the DRC contributed also to an increase in the transmission of sleeping sickness (23), river blindness (22, 24), and pneumonic plague (25–27) in addition to the spread of Ebola.

Destruction of healthcare infrastructure during wars resulting in weakened prevention and treatment programs can cause new strains of infectious diseases to emerge (28). Eradication of Guinea worm, river blindness, and polio programs have been disrupted resulting in challenges to delivering vaccines due to insecurity and conflict in countries experiencing war (28, 29).

As of 20 March 2023, around 8.1 million Ukrainians have moved to other European countries since 24 February 2022 after the attack by Russia on Ukraine, 5 million of whom were recruited into protection programs (30). Owing to the living conditions in the temporary shelters and the conditions they had faced during their movement, these displaced people are likely to acquire certain infectious diseases including EIDs or REIDs, and cases of EIDs or REIDs among this population are not unexpected (31).

There have been numerous reported outbreaks of conflict/war-associated EIDs/REIDs from a wide range of countries. Selected outbreak reports are presented in Table 2.

Natural disasters increase the risk of infectious disease outbreaks, including EIDs and REIDs, by damaging infrastructure and leading to displacement of populations (41). The population residing in natural disaster-prone areas has risen in most countries due to land shortage, urbanization, poverty, and population growth, and led to an increase in the public health impacts of natural disasters (42, 43).

A total of 432 catastrophic cases associated with natural disasters were recorded globally in 2021 though the average was 357 between 2001 and 2020 (44). The annual number of floods, which is the most common event among natural disasters, has increased from an average of 163 in the 2001–2020 period to 223 in 2021 (44). Additionally, globalization, climate change, and population movement cause natural disasters and their effects to become more complex (41).

Natural disasters are stratified into several categories by experts. Even though one classification divides natural disasters broadly into three general types (hydrometeorological, geophysical, and geo-meteorological disasters), any disaster in one category can have elements of another, for example earthquakes can cause avalanches and tsunamis, which can, in turn, cause flooding (45).

Hydrometeorological disasters such as floods from rains, hurricanes, cyclones, typhoons, and tsunamis are the most common natural disasters followed by geophysical disasters (Earthquakes, volcanic eruptions) while geo-meteorological ones are rarer than the others (45). Sometimes an emergence or re-emergence of an infectious disease agent can be, by itself, a natural disaster. Centre for Research on the Epidemiology of Disasters (CRED) classify these epidemics/pandemics as a biological sub-group of natural disasters (46). Plague outbreaks, especially the 14th century Black Death period and more recently the emergence of SARS-CoV-2 are the best examples. Even though these diseases cannot be attributed to any specific war or natural disaster they did have serious consequences on the control and prevention of other infectious diseases.

There have been numerous reported outbreaks of natural disaster associated EIDs/REIDs from a wide range of countries. Selected outbreak reports are presented in Table 3.

There are several causal factors in emerging and re-emerging infectious disease outbreaks in wars and natural disasters. Population movement and migration, environmental health disruption and ecological changes, displacement of domestic and wild animals, the collapse of health systems and disruption of disease control programs, inadequate surveillance and early warning and response systems, impaired laboratory services and diagnosis, and breakdown in infection control and treatment in effectiveness and development of drug resistance are the most accepted drivers of emerging and re-emerging infectious disease outbreaks in wars and natural disasters.

The impact of wars and natural disasters on emerging and re-emerging infectious disease outbreaks can be direct or indirect.

Waterborne, rodent-borne, and vector-borne diseases are the main groups of diseases associated with natural disasters. Emergence and increase in the incidence of waterborne diseases due to Campylobacter, Cryptosporidium, Escherichia coli, Giardia, Hepatitis A virus, Norovirus, Shigella, and Salmonella are generally reported from the affected areas (83).

It is widely recognized that natural disasters, especially hydrometeorological ones like floods, hurricanes, and cyclones, have the potential to lead to an increase in vector-borne diseases due to environmental factors (enlargement of vector habitats in number and area) enhancing vector population densities (84, 85). Existing mosquito breeding sites may be washed away at the beginning of flooding, but later on, new breeding sites for mosquitoes can occur due to overflows of rivers or rainfall and therefore enhance the risk of re-emergence of mosquito-borne diseases such as malaria, Chikungunya, and dengue (57, 85–87). There have been also emergence/re-emergence and outbreak reports of Chikungunya virus (CHIKV), Tahyna virus (TAHV), West Nile virus (WNV), and Japanese encephalitis diseases linked to flooding events (83, 88). Flooding can also cause a proliferation of disease vectors and microorganisms by changing the balance in ecosystems and the environment (43, 86). Mosquito-borne diseases, related to ecological changes which are suitable for mosquito breeding, also increase after earthquakes (49). Following the devastating earthquake in Ecuador in 2016, an important increase in the incidence of Zika Virus (ZIKV) was observed in the affected regions (89, 90). After the earthquakes in Iran in 2003, Cutaneous Leishmaniasis (CL) cases fluctuated, new foci emerged and the epidemiology of the disease changed (65, 66, 71, 91).

Mosquito-borne diseases were also reported during drought conditions. Drought causes reduce in the size of water bodies making them stagnant which leads to additional breeding places for mosquitoes (92). Also, the collection of rainwater due to inadequate water supply can increase the stagnant water sources (92). Drought-induced amplifications of the West Nile virus have been reported in the USA (55, 56). Malaria cases and deaths increased excessively due to the drought following an El Niño Southern Oscillation that occurred in Indonesia (68). Drought conditions caused numerous stagnant water puddles in rivers acting as mosquito breeding places (68, 93).

The risk of diseases transmitted by rodents such as leptospirosis and Hantavirus Pulmonary Syndrome increases as the probability of contact with bacteria and their animal hosts rise during heavy rainfall and flooding (94, 95). Flooding catalyzes the transmission of leptospirosis as a result of an increase in the number of infected rodents sheading leptopiras (57, 95). Insufficient garbage collection and management causing the spill of rubbish into streets after natural disasters can lead to an increase in rodent population (96, 97).

Rarely, unusual disease outbreaks such as coccidioidomycosis and scrub typhus have been observed following earthquakes. For example, an outbreak of acute pulmonary coccidioidomycosis related to exposure to high-level airborne dust following landslides after the 1994 earthquake occurred in southern California (52, 98). This outbreak illustrates the relationship between specific environmental conditions and the emergence of infectious diseases (52). After the massive earthquake in Nepal in 2015, scrub typhus emerged and outbreaks were reported from various parts of the country due to rodent infestation of the environment around temporary shelters which led people and rodents to be in close proximity (76, 77, 99).

Also, the destruction of infrastructure such as electricity, water supply, sewage disposal, and gas supply increases the risk of food poisoning and water-borne diseases after natural disasters (100, 101).

Outbreaks of leptospirosis and malaria were reported in Guatemala and Nicaragua after Hurricane Mitch in 1998. The number of diagnosed malaria cases was considerably higher than the number of cases diagnosed before the hurricane (54).

Melioidosis cases were reported in survivors of the tsunami which occurred in coastal areas of the Indian Ocean in December 2004 (72, 81). The risk of waterborne and vector-borne diseases increases due to physical changes in the environment in tropical cyclones, followed by floods and sea surges (102).

Findings from previous El Nino episodes show that rains due to El Nino that result in increased rodent density heightens the risk of human contraction of Hantavirus pulmonary syndrome (103). A previously undescribed disease, subsequently identified as a previously unknown hantavirus (Bunyaviridae), emerged in the Southwest USA in 1993 after El Nino Southern Oscillation events the year before, which had allowed excess precipitation, warmer seasons, and plenty of food for rodents (21, 103). Also, a statistically significant association was demonstrated between malaria epidemics and El Niño most probably due to widespread flooding during El Niño (104).

Malnutrition, which increases vulnerability to EIDs/REIDs, is a common situation observed both in wars and natural disasters, especially in long-term events such as droughts. Food security created by conflicts, civil strife, and extreme weather conditions—particularly repeated droughts—is the main reason for malnutrition in less developed countries (190, 191). According to World Food Programme around 258 million people were acutely food-insecure in 2022 in 58 countries (191). The Greater Horn of Africa including Djibouti, Ethiopia, Kenya, Somalia, South Sudan, Sudan, and Uganda is the worst food-insecure region for decades. A rising trend in food insecurity is being currently observed in the region due to extreme weather events such as drought and flooding, conflict and instability, high food prices, and the socioeconomic impacts of COVID-19 (192, 193).

Food insecurity can cause impairment in the immune system due to malnutrition making people more susceptible to infectious diseases and generally associated with population displacement, overcrowding, the collapse of health system measures such as vector control and vaccination programs, and poor sanitation which poses the risk of infectious diseases outbreaks (192, 194). Particularly women, children, and people living with chronic diseases like human immunodeficiency virus (HIV) and tuberculosis are vulnerable due to food insecurity (192). There are several well-established studies confirming the bidirectional link between HIV and food insecurity: Food insecurity may increase susceptibility to HIV, and HIV may lessen the HIV patient’s ability to make and/or aquire food (190).

Food insecurity can also contribute indirectly to shape, re-shape, or enhance vulnerabilities to EIDs/REIDs, especially in less developed countries. For example, the risk of contracting HIV/AIDS is much higher in women than men now even though men were more likely to contract the disease when the disease emerged (190, 195). The reason for this inequality is a mixture of several biological, cultural, economic, and social factors, especially in less-developed countries. A study assessing the data of 91 less-developed countries indicated that drought-induced food insecurity was directly and indirectly associated with women’s vulnerability to HIV. Women generally were more likely to get a lower amount of food than men, which causes nutrient deficiencies leading to increased susceptibility to infection. Additionally, food insecurity was also associated with social vulnerabilities such as a decrease in women’s access to health facilities and forced engagement in risky sexual behaviors, etc. (190).

A systematic review analyzing 111 studies even suggested an indirect relationship between drought and HIV treatment in African countries. Changes in livelihood and economic conditions due to drought-induced water and food insecurity and population movement appeared to affect HIV treatment adherence according to the review (196). Another review’s findings identified a relationship between food insecurity and initiation of antiretroviral therapy (ART) and treatment adherence (197).

The impact of wars and natural disasters on EIDs/REIDs varies depending strongly on the level of underlying fragility or vulnerability in the affected country or society. Less developed countries are significantly more vulnerable to disasters due to having a large number of vulnerable populations, limited response capacity, weak health systems, etc. (198). The inadequate health infrastructure and insufficient human and financial resources before, during, and after wars and natural disasters in less developed countries may increase the vulnerability, and EIDs/REIDs incidences such as malaria and HIV reach epidemic proportions (198). In contrast, EIDs/REIDs outbreaks can be confined within a particular area in developed countries as a result of strong health systems and their coping capacity with disasters.

However, development is not a guarantee of ‘invulnerability,” different levels of vulnerabilities within societies or populations even in developed countries can also be observed (198). The older adults and poor black populations were a significantly high proportion among victims of Hurricane Katrina in New Orleans in 2005 (199, 200). Similarly, the young, the older adults, and women were affected more than others during the earthquake in Italy in 2009 (200).

Conflicts also can generate additional vulnerabilities during natural disasters owing to weakening the respond capacity of countries. Prevailing vulnerabilities in conflict situations are generally aggravated by disasters and conflicts may worsen the impact of disasters. There are some studies suggesting that conflict has been higher in drought situations (201).

In a systematic review analyzing 132 studies, population displacement was the most frequently reported risk factor in infectious disease outbreaks followed by water, sanitation, and hygiene (WASH), housing, and vector/animal after disasters (202). In another review crowded conditions, forced displacement, poor quality shelter, poor water, sanitation and hygiene, lack of healthcare facilities, and lack of adequate surveillance were found to be the key risk factors cascades for communicable disease outbreaks in complex humanitarian emergencies (203). All these factors exacerbate the pre-existing vulnerability and increase the probability of post-disaster infectious disease outbreaks (202).

The extent (geographically, number of people affected, duration, etc.) of the disaster is also important in the context of the coping capacity of the country. For example, the devastating earthquake on 6 February 2023 with a 7.8 magnitude followed by 5,700 aftershocks directly affected 11 provinces living 15 million people in Türkiye and 11 million families in the Syrian Arab Republic (204, 205).

A Generic National Action Plan covering risk assessment, a mechanism for action, determination of roles and responsibilities of each sector, establishment of a coordination mechanism, etc. should be developed. Each province/region should prepare its own plan of action considering local conditions such as epidemiology of EIDs/REIDs, vector and reservoir species and their distribution, response capacity and additional needs, etc.

Even though there have been common risks and drivers for EIDs/REIDs in wars and natural disasters, they have idiosyncratic risks. For example, security is a great problem in wars while the continuity of a natural disaster itself, such as in an earthquake poses, additional risks in implementing control measures.

In order to increase response capacity, planning and investments including awareness raising additional to finance are needed for disaster preparedness and early warning. The establishment or adaptation of the current EIDs/REIDs Surveillance and Early Warning System covering the probable effects of wars and natural disasters on EIDs/REIDs is a prerequisite for recognizing disease clusters/outbreaks, and monitoring and evaluating control interventions. Being an important part of a strong surveillance system, enhanced laboratory services should be planned to include mobile even field laboratories which are capable of sustaining their work under inappropriate conditions in order to provide rapid diagnosis of diseases. Maintenance of surveillance and control activities, waste management services, and food security are of great importance during and after wars and natural disasters in order to reduce their impact on EIDs/REIDs. Given the risk of EIDs/REIDs is much higher in overcrowding and unsanitary conditions, priority should be given to the displaced population and refugees living in temporary shelters and regular screenings can be performed especially for vector-borne diseases such as malaria and leishmaniasis.

Free roaming animals, whether domestic/wild or stray/owned, should definitely be considered, and special control interventions should be implemented as most of the EIDs/REIDs have zoonotic origin. Planning of vector and rodent control is of great importance too. The risk of occurrence of natural disasters and, EIDs/REIDs following natural disasters and during conflict conditions are increasing due to climate change which contributes to creating environmental changes especially favorable for vector-borne diseases.

Therefore, the impact of wars and natural disasters on EIDs/REIDs should be taken seriously. Recommended actions in order to prevent and control EIDs/REIDs in wars and natural disasters are summarized in Table 4.

ST, AT-O, and EA: conceptualization, writing—review and editing, and visualization. ST and AT-O: methodology. AT-O and EA: validation. ST: resources and writing—original draft preparation. EA: supervision. All authors contributed to the manuscript development and approved the final version to be published.

This study was supported by TOBB-ETU.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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