Disease X is not an actual disease. It’s a placeholder name the World Health Organization uses to represent a hypothetical future epidemic or pandemic caused by a pathogen not yet known to infect humans. The WHO added Disease X to its priority research list in 2018, two years before COVID-19 proved the concept was more than theoretical. The idea is simple: if scientists only prepare for known threats, they’ll always be caught off guard by new ones.
Why the WHO Created a Name for Something That Doesn’t Exist
The WHO maintains a list of diseases that pose the greatest public health risk and need urgent research investment. That list includes known threats like Ebola, Zika, and MERS. Disease X sits alongside them as a reminder that the next major outbreak could come from a virus or bacterium no one has encountered before.
As WHO scientists have explained, there is no virus or bacteria circulating right now called Disease X. The term exists purely to drive preparedness. Scientists know which types of viruses and bacteria to watch, but there are thousands of candidates. Disease X gives them a shorthand for talking about the unknown without having to specify which pathogen will eventually cause trouble. When discussing the pathogen itself rather than the illness it would cause, researchers sometimes use the term “Pathogen X.”
Which Virus Families Are Most Likely Candidates
Not all viruses carry equal pandemic potential. Research published in Open Forum Infectious Diseases identifies six viral families as the highest priority based on two key traits: they spread efficiently through the air between humans, and we lack effective treatments or vaccines for many species within them.
- Orthomyxoviridae, the family that includes influenza viruses
- Coronaviridae, which gave us SARS, MERS, and COVID-19
- Paramyxoviridae, home to measles and Nipah virus
- Picornaviridae, a large family that includes enteroviruses
- Pneumoviridae, which includes RSV
- Adenoviridae, common respiratory viruses that occasionally cause severe outbreaks
Each of these families has already produced viruses that spread widely in human populations. The concern is that a new species or variant within one of them could emerge with a combination of high transmissibility and severity that existing tools can’t handle.
What Drives New Pathogens to Emerge
Most emerging infectious diseases in humans originate in animals, particularly wildlife, and then jump the species barrier. This process, called spillover, is becoming more frequent because of how humans are reshaping the planet.
Deforestation is one of the biggest drivers. When tropical and subtropical forests are cleared for agriculture, people and livestock move into areas where they encounter wildlife they previously had no contact with. Animals stressed by habitat loss may shed viruses at higher rates. The combination of new contact and increased viral shedding creates ideal conditions for a pathogen to make the leap into humans.
Commercial wildlife markets and trade compound the problem. When wild animals are captured, transported, and sold alongside domestic animals, viruses from different species mix in close quarters. This creates opportunities not just for spillover but for viruses to swap genetic material and produce entirely new strains. Climate change adds another layer of risk by shifting where animals live, pushing species into new territories and creating contact between populations that previously had no overlap.
How the World Is Preparing
Preparedness for Disease X falls into three main categories: surveillance to detect new threats early, platform technologies to respond quickly, and international agreements to coordinate the response.
Genomic Surveillance
The WHO’s International Pathogen Surveillance Network (IPSN) brings together organizations across countries and income levels to monitor the genetic material of circulating pathogens. By sequencing the genomes of viruses, bacteria, fungi, and parasites, scientists can track how pathogens evolve, spot unusual mutations, and detect new threats before they spread widely. This approach proved its value during COVID-19, when genomic surveillance allowed researchers to identify new variants in near real time.
The 100 Days Mission
Perhaps the most ambitious preparedness goal is what’s known as the 100 Days Mission, led by the Coalition for Epidemic Preparedness Innovations (CEPI). The target: within 100 days of identifying a new pandemic threat, have accurate rapid diagnostic tests, an initial set of treatments, and authorized vaccines ready for large-scale manufacturing. That’s just over three months from recognition to response, a timeline that would have seemed impossible before the COVID-19 vaccine effort demonstrated how fast development could move.
CEPI funds a portfolio of projects specifically tagged as Disease X preparedness. These aren’t vaccines against a specific virus. They’re platform technologies, flexible systems that can be quickly adapted once a new pathogen is identified. Current investments include research into nanoparticle delivery systems, mRNA delivery platforms, and other approaches designed to be repurposed rapidly. Individual grants range from hundreds of thousands to several million dollars each.
The Pandemic Agreement
In May 2025, WHO member states formally adopted the world’s first Pandemic Agreement by consensus. The agreement includes provisions for a Pathogen Access and Benefit Sharing system, designed to ensure that when a dangerous new pathogen is identified, genetic sequences and biological samples are shared quickly while the benefits of any resulting vaccines or treatments are distributed fairly. Under the agreement, pharmaceutical manufacturers participating in the system would make 20% of their real-time production of vaccines, therapeutics, and diagnostics available to WHO during a pandemic emergency. The agreement needs 60 countries to ratify it before it takes legal effect.
Why COVID-19 Changed the Conversation
When the WHO added Disease X to its priority list in 2018, it was a theoretical exercise. Then SARS-CoV-2 emerged in late 2019, a novel coronavirus that fit the Disease X profile almost exactly: a previously unknown pathogen causing a serious international epidemic. The pandemic killed millions, disrupted economies worldwide, and exposed gaps in every country’s preparedness systems.
COVID-19 turned Disease X from an abstract concept into a lived experience. It demonstrated that a new respiratory virus could circle the globe in weeks, that healthcare systems could be overwhelmed even in wealthy countries, and that developing and distributing vaccines, while faster than ever before, still took long enough for the virus to cause enormous harm. Every current preparedness initiative, from the 100 Days Mission to the Pandemic Agreement, is shaped by lessons from that experience.
The core message behind Disease X remains unchanged: the question is not whether another unknown pathogen will emerge, but when. The investments being made now are designed to ensure the next time is less devastating than the last.