The reemergence of diseases presents a significant challenge to global public health, marking a dynamic and ever-changing landscape of biological threats. These are not new diseases, but rather known illnesses that reappear or increase in incidence after a period of decline or near eradication. Understanding the factors behind their resurgence is paramount for developing effective strategies to protect populations worldwide.
Understanding Reemergence
Reemerging diseases are known illnesses that reappear or increase in incidence after a period of decline or near eradication. This contrasts with “emerging” diseases, which are newly identified infections or known agents spreading to new locations. For instance, HIV was an emerging disease in the 1980s, while tuberculosis is now considered reemerging due to drug resistance. Their resurgence indicates a failure to maintain control over a pathogen.
Drivers of Reemergence
A multitude of interconnected factors contribute to the reemergence of biological threats. Antimicrobial resistance is a primary driver; the overuse and misuse of antibiotics have accelerated the evolution of resistant microorganisms, making once-treatable infections challenging to manage. This resistance extends to other drugs, including those for malaria.
Climate change and environmental shifts also play a role, altering vector habitats and disease transmission patterns. Rising temperatures and changes in precipitation can expand the geographic range of disease-carrying insects like mosquitoes and ticks. Warmer temperatures can also accelerate the developmental cycle of mosquitoes, increasing their biting rates and disease transmission. Human encroachment into natural habitats, driven by population growth and agricultural expansion, further increases contact between humans and wildlife, facilitating the spillover of zoonotic pathogens.
Globalization and increased travel and trade enable the rapid international spread of pathogens, as infected individuals or contaminated goods can quickly move across borders. Socio-economic factors, such as poverty, inadequate sanitation, and high population density, create environments where diseases can easily spread and persist. Unplanned urbanization often leads to crowded living conditions and poor hygiene, fostering the resurgence of diseases like cholera and tuberculosis.
Breakdowns in public health infrastructure or vaccination programs further weaken defenses against pathogens. Reduced funding for public health can lead to decreased diagnosis and treatment efforts, contributing to the reemergence of drug-resistant strains. Declining vaccination rates reduce herd immunity, allowing diseases to resurface in susceptible populations. Pathogen evolution, through mutation or genetic exchange, also allows them to adapt to new hosts, evade immune responses, or develop drug resistance, constantly challenging control efforts.
Notable Reemerging Cases
Measles, once eliminated in the United States in 2000 due to high vaccination rates, has seen a resurgence. This reemergence is largely attributed to declining childhood vaccination rates, with outbreaks often occurring in communities where immunization coverage falls below the 95% threshold needed for herd immunity. Measles is highly contagious.
Tuberculosis (TB) is a reemerging threat, primarily driven by drug-resistant strains and socio-economic factors. Though drugs dramatically reduced cases decades ago, TB reemerged alongside the HIV epidemic and is increasingly resistant to traditional antibiotics. Multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) strains are now much harder to cure. Overcrowding and poor living conditions in urban areas also facilitate its spread.
Polio, once on the verge of global eradication, has reemerged in specific regions. This resurgence is linked to drops in immunization rates, partly due to the COVID-19 pandemic’s disruption of vaccination campaigns. In areas with persistently low vaccination coverage, weakened vaccine-derived poliovirus can circulate and mutate, regaining the ability to cause paralysis. Cases of vaccine-derived poliovirus type 2 (cVDPV2) were reported globally in 2022, primarily in countries like the Democratic Republic of Congo, Yemen, and Nigeria.
Dengue fever, a mosquito-borne viral disease, has expanded its global reach. Climate change, leading to higher temperatures and increased rainfall, creates ideal breeding grounds for the Aedes aegypti mosquito, the primary vector. Uncontrolled urbanization and human mobility also contribute, as more people live in dense urban areas where mosquitoes thrive, and increased travel facilitates the virus’s spread to new regions. Countries like Brazil, Peru, and Bangladesh have experienced record-high dengue cases following these environmental and societal shifts.
Global Preparedness and Response
Addressing reemerging threats requires a global strategy. Surveillance and early warning systems are foundational, involving the continuous collection, analysis, and interpretation of health data to detect outbreaks quickly. This includes passive reporting from health facilities and active investigations, alongside systems that leverage internet and information technologies for rapid intelligence gathering. The World Health Organization (WHO) plays a central role in monitoring global public health threats and disseminating alerts.
Vaccination campaigns and public health initiatives remain tools for prevention and control. Sustained and high vaccination coverage, ideally at least 95% for diseases like measles, is necessary to achieve herd immunity and prevent outbreaks. Efforts also focus on improving vaccine access, especially in resource-poor communities, and addressing vaccine hesitancy through public education.
Research and development (R&D) of new diagnostics, treatments, and vaccines are needed to stay ahead of evolving pathogens. This includes investing in novel vaccine platforms, such as mRNA technology, and developing rapid, accurate diagnostic tools for early detection, particularly for zoonotic diseases. Efforts also focus on improving treatment options, including new antimicrobial agents to combat resistance.
International cooperation and organizations like the WHO are central to a coordinated global response. These bodies facilitate the sharing of information, resources, and expertise across borders. The “One Health” approach recognizes the interconnectedness of human, animal, and environmental health, acknowledging that most emerging human pathogens originate in animals. This integrated approach involves multisectoral collaboration among medical, veterinary, and environmental disciplines to prevent and control zoonotic diseases. The Quadripartite, comprising the Food and Agriculture Organization of the United Nations (FAO), the United Nations Environment Programme (UNEP), the WHO, and the World Organisation for Animal Health (WOAH), developed a Joint Plan of Action (2022–2026) to collectively tackle health threats.