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The resurgence of infectious diseases
- Page updated onMarch 17, 2025
The 21st century has witnessed a wave of major epidemics since its beginning, including outbreaks of Severe Acute Respiratory Syndrome (SARS), Ebola, yellow fever, Zika, and, of course, the COVID-19 pandemic. These epidemics have been driven by demographic, technological, and climate changes, which have even reversed previous progress in water, sanitation, hygiene, and access to healthcare services.
Since the COVID-19 pandemic in 2020, these trends have worsened. Many infectious diseases have resurged, with causes including the pandemic’s impact on vaccination coverage and the effect of reduced social interactions on immunity. Additionally, this resurgence overlaps with the increased risk of infections due to the rise in the number, duration, and impact of humanitarian crises. In 2023, of the 36 countries with a humanitarian response plan, 19 experienced an infectious disease outbreak. Responding to epidemics in such complex contexts is often particularly challenging.
Table of contents:
What has caused the resurgence of infectious diseases?
A series of recent changes have altered the risks of infectious diseases globally. These interconnected changes include climate, demographic, and technological shifts, along with the impact of emerging and re-emerging diseases themselves on the resurgence of others.
Climate change
On one hand, climate change causes significant shifts in the distribution patterns of pathogen reservoirs and vectors, as well as changes in susceptibility to infections. For example, changes in environmental conditions may lead a reservoir animal species to migrate over long distances, potentially fostering new contacts with different animal species that may, in turn, infect humans.
Additionally, temperature changes can increase risks linked to seasonality or the reproductive capacity of certain pathogens, such as the bacteria causing cholera. At the same time, increased air pollution raises susceptibility to respiratory viruses. Other effects, such as climate change’s impact on vector life cycle characteristics related to pathogen transmission, are still not well understood.
Demographic changes
During this period, significant demographic changes have also occurred, such as the increase in population, migration, and concentration in urban areasintroduction of pathogens in destination regions or countries. This is especially concerning where there is inadequate access to quality healthcare services or a lack of dignified living conditions.
In addition, the rapid increase in urbanization rates has contributed to the spread of diseases such as dengue and Zika, transmitted by mosquitoes that are well adapted to urban areas. This has also played a role in the increased transmission of respiratory infections such as influenza or COVID-19.
The increase in travel and movement of people
Some technological changes, such as improvements in healthcare, water, and sanitation, reduce the risk of infections. In contrast, others, like the constant increase in travel (and the speed at which it occurs) of people and the transport of plants, animals, and derived products, facilitate their spread. As a result, today, vectors and microorganisms can travel farther and faster than ever before in our history. This can lead to many new interactions between pathogens and hosts or introduce local transmission in new geographical areas.
The impact of some infections on the reemergence of others
Finally, the emergence or re-emergence of certain diseases also increases the risk of others. Examples of this include the syndemic relationship between HIV/AIDS and tuberculosis. Another example is the impact of Ebola outbreaks on the rise in measles cases, resulting from the collapse of healthcare services in affected countries.
In this regard, it is important to highlight the impact of the COVID-19 pandemic on nearly all vaccination coverage. The pandemic led, in many countries, to the diversion of resources from routine vaccination to the fight against COVID-19, the emergence of new barriers to access and use of services, and the distrust generated among the population towards vaccination. As a result, the reduction in vaccination coverage for many diseases has resulted in enormous infectious outbreaks as these diseases have re-emerged.
Risk of infectious diseases is higher in humanitarian settings
In humanitarian crises, the effects of recent demographic, technological, climatic, and epidemiological changes that have increased the risk of infectious diseases worldwide are also felt. However, in these contexts, there are additional specific risks for the rise of infectious diseases. These include the effects of conflicts, forced displacements, natural disasters, and the collapse of public systems and services. In these contexts, there is a higher prevalence of food insecurity, malnutrition, poor housing conditions, water, sanitation, and hygiene, and disruption or deterioration of preventive and curative health services. All of this increases the risk of epidemics, which can further exacerbate crises or contribute to causing them.
Risk of infection increases when health services deteriorate acutely
A complex emergency can lead to the interruption of health service provision or prevent entire populations from accessing available services. The entire health system may be affected, resulting in reduced capacities and public health functions.
In complex humanitarian crises, the interruption of vaccination programs can result in a poor population immunity against numerous diseases. This can be especially severe for diseases like measles, rubella, mumps, or polio, which require a high percentage of vaccination coverage to achieve herd immunity. The problem, however, is not limited to this. Lack of access to prenatal and childbirth care can increase the risk of vertical transmission infections such as neonatal tetanus or epatitis B. Similarly, the lack of availability or access to curative health services can be particularly severe when there are cases of infectious diseases that require urgent treatment. These include cholera or rotavirus diarrhea, meningitis, rabies, or tetanus, among others.
In addition, a conflict can weaken the health information system and epidemiological surveillance mechanisms of a context, reducing the system's capacity to detect cases of new infections early and respond to epidemics. This, for example, has been a determining factor in recent outbreaks of Ebola in the Democratic Republic of the Congo and West Africa.
The displaced or host population is exposed to new pathogens
In crises, forced displacements and large population movements can occur, exposing both displaced and host populations to new risks. The displaced population may have a high prevalence of infectious diseases to which the host population is not immunized, or vice versa. As a result, the risk of introduction or reintroduction of these infections increases. For example, the incidence of malaria can multiply if an immunologically naive population is displaced to an endemic area. The same can happen if many people with subclinical infections arrive in densely populated urban areas.
The acute deterioration of living conditions and hygiene increases vulnerability to infections
Crises often have a significant negative impact on living conditions. This can increase susceptibility to infections, especially in certain climates or seasons of the year. Examples of this occur when there is a large destruction of housing and water and sanitation infrastructure, or when there is a sudden increase in population density in urban areas or camps for displaced people. In both cases, the available services may be insufficient to prevent outbreaks of infectious diseases.
Crowding increases the risk of respiratory-transmitted diseases such as diphtheria, measles, meningitis, tuberculosis, or pneumococcal infection. This susceptibility also increases, in part due to the effects of climate change, during cold or dry periods, if the housing and shelter conditions for a displaced population or those affected by a complex crisis are poor.
The possibility of transmitting some fecal-oral transmitted diseases, such as polio and cholera, also increases with overcrowding. Additionally, issues with access to water, sanitation, and hygiene predispose individuals to all types of diarrheal and fecal-oral transmitted diseases, including those already mentioned as well as others like hepatitis A and E or typhoid fever. The effects of climate change also impact these pathologies during the rainy season or in flood-induced crises.
Epidemiological patterns change in humanitarian crises
The changes in disease burden are also common in these contexts and can have serious consequences on the incidence of certain infections. For example, many humanitarian crises result in an increase in cases of acute malnutrition and chronic malnutrition. This, in turn, increases the comorbidity of diseases such as whooping cough, measles, pneumococcal disease, tuberculosis, or typhoid fever.
The increased risk of sexual and gender-based violence or sexually transmitted infections such as HIV/AIDS is also related to an increase in infection by human papillomavirus and hepatitis B. It is also linked to co-infection with tuberculosis and chickenpox.
Re-emergence of infectious diseases in emergencies may have global impact
We cannot forget that humanitarian contexts are not isolated or closed off. The emergence of cases of certain diseases with epidemic potential, such as measles and polio, can cause epidemics that cross borders. When this happens, they can reverse the progress made by significant global efforts for their elimination and eradication.
Recent outbreaks of infectious diseases in humanitarian settings
There are many recent examples of infections that resurge in humanitarian contexts. Among others, these include cholera, polio, diphtheria, yellow fever, meningococcal meningitis, and measles.
Cholera in many countries since 2022
In just 2022 and 2023, there were outbreaks of cholera in Bangladesh, Burundi, Cameroon, the Democratic Republic of the Congo, Ethiopia, the Philippines, Haiti, Kenya, Lebanon, Malawi, Mozambique, Nigeria, Pakistan, Somalia, South Sudan, Sudan, Syria, and Yemen. During this period, there were reported cases in over 40 countries, while in the five previous years, there were reported cases in only 20 countries.
This resurgence of cholera has far exceeded the response capacity of countries and the international community. The global stockpile of oral cholera vaccine, established at 5 million doses (and completely depleted for months), along with the annual global manufacturing capacity (less than 50 million new doses at full capacity), has been insufficient to meet the enormous demand for vaccines, which in two years has surpassed the total demand of the entire previous decade.
Polio in Gaza in 2024
Polio is a very serious disease for which there is no treatment, and it can cause damage to the nervous system, resulting in paralysis and even death. Despite the recent eradication of type 2 in 2015 and type 3 in 2019, and the fact that wild poliovirus type 1 is only still found in Pakistan and Afghanistan (it was eradicated in Africa in 2020), the disease has not yet been completely eradicated.
Gaza, in 2024, had been polio-free for 25 years and had close to 95% vaccination coverage in 2022. However, the disease reappeared as a result of 10 months of intense Israeli bombardment that caused massive destruction of water and sanitation infrastructure.
Diphtheria has reappeared in African countries
Diphtheria is a respiratory infection for which there has been a vaccine for nearly a century, and it can be lethal in up to 30% of cases among unvaccinated individuals without treatment. This condition is resurging in countries such as Niger, Mauritania, Guinea, South Africa, and especially Nigeria. While only about 29,000 cases of this disease were reported across the African continent between 2013 and 2022, in Nigeria alone, 18,000 suspected cases and nearly 600 deaths were reported in 2023.
A large outbreak of yellow fever shook Angola and the Democratic Republic of Congo in 2016
In 2016, Angola and the Democratic Republic of the Congo experienced an unprecedented urban outbreak of yellow fever in the region. With a significant public health risk due to its potential transmission to areas in Asia, the outbreak required more than 28 million vaccine doses for its control. This amount, in fact, far exceeded the global stockpile and even the capacity to manufacture new doses.
The circumstances of the outbreak forced the use of fractional doses of vaccine as the only feasible possibility. They also prompted the development of a new strategy for yellow fever elimination 2017-2026. This strategy has included the manufacture of 1400 million new doses to increase population immunity levels in the 40 most at-risk countries (13 in the Americas and 27 in Africa). This also minimizes the risk that the disease could reach other countries in Asia.
Meningitis has recently hit Niger and Nigeria with great force
Meningitis, a serious infectious disease caused by various viruses, fungi, and bacteria, continues to pose a significant threat to public health. Among the different bacteria that can cause it, several are preventable through vaccines. Some of these vaccines have been available for several years, while others are still under development.
In 2023, WHO prequalified and began recommending the use of a new vaccine produced in Nigeria against the five strains of meningococcal meningitis with the highest epidemic potential. This vaccine has already been used in Nigeria for an outbreak of meningococcal meningitis with 1742 cases and 153 deaths. Following this, it could also be used in May 2024 in Niger, where the number of reported cases reflected an enormous increase since the previous year.
Measles has increased in incidence, while vaccination coverage has declined
Measles, a highly contagious viral disease, caused over 2.5 million deaths annually in the 1980s before widespread vaccination. Its incidence is often used as an indicator of inequality, as it particularly impacts displaced populations in densely populated areas with low vaccination coverage.
Its overall vaccine coverage (with one dose) declined from 86% in 2019 to 81% in 2021 as a result of the COVID-19 pandemic. At the same time, the number of measles cases has increased from 159,000 in 2020 to 664,000 in 2022.
Infectious diseases
External links
- ACAPS, 2024. The global resurgence of cholera aggravated by vaccine shortage and El Niño.
- Lubanga, 2024. Addressing the re-emergence and resurgence of vaccine-preventable diseases in Africa: A health equity perspective.
- BMJ, 2024. Analysis reveals global post-covid surge in infectious diseases.
- Marou, 2024. The impact of conflict on infectious disease: a systematic literature review.
- OCHA, 2023. Global Humanitarian Overview 2024. Disease outbreaks and converging risks threaten weak and recovering health systems.
- Baker, 2021. Infectious disease in an era of global change.
- OMS, 2017. Vaccination in acute humanitarian emergencies: a framework for decision making.
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