An emerging disease is an infectious disease that is either entirely new to humans, spreading to new populations for the first time, or recently identified as a distinct illness caused by a specific pathogen. Three out of every four new or emerging infectious diseases in people come from animals, making the boundary between wildlife and human health one of the most important factors in understanding how these threats develop.
The term also covers re-emerging diseases: infections that were once controlled but are now surging again. Many experts treat re-emerging diseases as a subcategory of emerging diseases, since the underlying forces driving both overlap significantly.
Three Ways a Disease Qualifies as “Emerging”
Not every new headline about a virus means a brand-new pathogen has appeared. The National Institutes of Health defines three distinct paths to emergence. First, a disease may truly be new to humans, meaning no prior human infection has ever been documented. This is actually the rarest scenario. Second, a disease may have existed for years but only affected small, isolated groups of people before spreading more widely. HIV/AIDS and Ebola both followed this pattern, circulating in limited communities before reaching broader populations. Third, a disease may have been present throughout human history but was only recently pinpointed as a distinct infection with a specific cause. Lyme disease is a classic example: the symptoms existed long before scientists identified the bacterium behind them in the early 1980s.
Why Previously Controlled Diseases Come Back
Re-emerging diseases are infections that once declined dramatically but have returned as serious public health threats. Tuberculosis is one of the starkest examples. After decades of progress, TB began re-emerging globally over the past 30 years, partly because the spread of HIV weakened immune systems and created ideal conditions for TB transmission. Drug-resistant strains of TB have compounded the problem.
Cholera, once thought to be fading from the modern world, continues to re-emerge in parts of Africa, Asia, and South America, particularly where clean water infrastructure breaks down. Dengue fever has expanded into new regions across Central America, Latin America, and Southern Asia as mosquito habitats shift with changing climates. When the proportion of immune individuals in a population drops below a critical threshold, whether through declining vaccination rates, war, or poverty, diseases that seemed conquered can rapidly return.
What Drives New Diseases to Appear
Emerging diseases don’t appear randomly. They arise when a specific mix of conditions allows a pathogen to gain a foothold in a vulnerable population. Researchers have identified over a dozen recurring drivers, but several stand out for their outsized influence.
- Land use and ecosystem changes. Deforestation, mining, and agricultural expansion push humans into closer contact with wildlife, increasing the chances of a pathogen jumping species. More than 6 out of every 10 known infectious diseases in people can spread from animals.
- Climate and weather. Warming temperatures expand the geographic range of mosquitoes, ticks, and other organisms that carry disease. Regions that were once too cold for certain vectors now support year-round transmission.
- Human demographics and behavior. Urbanization concentrates people in dense environments where respiratory and waterborne diseases spread efficiently. International travel and commerce move pathogens across continents in hours.
- Poverty and social inequality. Communities with limited healthcare infrastructure, poor sanitation, and food insecurity face disproportionate risk. These populations are often where outbreaks begin and where they’re hardest to contain.
- Breakdown of public health measures. Vaccine hesitancy, underfunded surveillance systems, and political instability all create gaps that pathogens exploit. War and famine displace populations and destroy the health systems that keep diseases in check.
Microbial adaptation also plays a role. Bacteria and viruses mutate constantly, and some mutations allow a pathogen to infect new host species, resist existing drugs, or spread more efficiently. The emergence of multidrug-resistant tuberculosis is a direct result of this evolutionary pressure combined with inconsistent treatment practices.
The Animal Connection
The majority of emerging infectious diseases are zoonotic, meaning they originate in animals before crossing into humans. According to the CDC, roughly 75% of new or emerging infections follow this path. SARS-CoV-2, Ebola, HIV, avian influenza, Nipah virus, and Zika all trace back to animal reservoirs.
This reality has reshaped how public health organizations approach disease prevention. The One Health framework, endorsed by the World Health Organization, treats human health, animal health, and environmental health as deeply interconnected. Rather than waiting for a pathogen to appear in hospitals, One Health aims to monitor wildlife populations, track environmental changes, and detect unusual disease patterns at the source. This integrated approach covers the full spectrum from prevention to detection, preparedness, and response.
How New Pathogens Are Found
Scientists discover an average of more than two new species of human virus every year. In any given year, the number ranges from zero to six. That steady pace of discovery reflects both the sheer diversity of the microbial world and dramatic improvements in detection technology.
The biggest leap has come from a technique called metagenomic sequencing, which can scan a patient sample for genetic material from any pathogen (bacteria, viruses, fungi, parasites) without needing to guess what you’re looking for first. This “hypothesis-free” approach has proven critical during outbreaks. During a hemorrhagic fever outbreak in the Democratic Republic of Congo, researchers used this method to identify a completely novel virus directly from patient specimens. The same technology revealed that Zika virus had been silently spreading in Brazil for months before doctors recognized the first clinical cases.
Portable sequencing devices have made it possible to do this work in the field rather than shipping samples to distant laboratories. These platforms were deployed during Ebola, Zika, and COVID-19 outbreaks. In one hospital study, nanopore sequencing identified respiratory pathogens in under six hours, with same-day results achieved in 86% of samples. That speed matters enormously when a new pathogen is spreading and public health officials need to know what they’re dealing with.
Global Surveillance Systems
Detecting an emerging disease early enough to contain it requires constant monitoring. The WHO operates a global surveillance system that screens for public health threats 24 hours a day, 365 days a year. When something unusual appears, the organization can activate the Global Outbreak Alert and Response Network (GOARN), a coalition of technical institutions that pool expertise and resources during outbreaks of international importance.
For remote or resource-limited settings, the WHO developed the Early Warning, Alert and Response System (EWARS), designed to detect and track health events in places where traditional surveillance infrastructure may not exist. Communication happens through formal International Health Regulations procedures, published Disease Outbreak News bulletins, and social media channels that can reach affected communities quickly.
The Economic Toll
Emerging diseases cause damage well beyond the direct health effects of illness and death. The 2003 SARS outbreak in East Asia illustrated how an infectious disease can trigger severe economic disruption even when the total number of cases is relatively small. Fear of infection, travel restrictions, and business closures rippled through economies far beyond the regions with active transmission.
A similar pattern played out during the 1994 plague outbreak in Surat, India, where economic losses vastly exceeded what the case count alone would suggest. This “SARS-type economic effect,” where behavioral changes and policy responses amplify the financial impact, is now recognized as a defining feature of emerging disease outbreaks. It explains why governments and international organizations invest heavily in early detection and rapid response: catching an outbreak early is orders of magnitude cheaper than managing one that has spiraled.
Pathogens on the Watch List
The U.S. National Institute of Allergy and Infectious Diseases maintains a regularly updated list of priority pathogens that pose significant threats. The list spans bacteria, viruses, fungi, and parasites, and it evolves as new threats appear. Recent additions reflect shifting concerns: Cache Valley virus, Langya virus, Mayaro virus, and Oropouche virus were all added in 2024, signaling growing attention to vector-borne diseases expanding into new territories. Established threats like highly pathogenic coronaviruses (including SARS-CoV-2), Ebola, Marburg, Nipah, and avian influenza remain priorities alongside longer-standing concerns like anthrax, plague, and drug-resistant tuberculosis.
The breadth of this list underscores a core reality about emerging diseases: they are not a single problem with a single solution. They emerge from the intersection of biology, ecology, human behavior, and economics, and managing them requires watching all of those fronts simultaneously.