Over the last few weeks, our newsfeeds have been once again dominated by various outbreaks from across the globe.

First there was the deadly hantavirus outbreak aboard the cruise ship MV Hondius. Three people died after traveling on the cruise ship, with two of them confirmed to have had the virus. Overall, there have been 10 hantavirus cases (eight confirmed and two probable) linked to the outbreak.

Just as the cruise passengers were being evacuated from the vessel and airlifted from Cape Verde to their home countries to quarantine, some 6,000 kilometers away, in Ituri province of the Democratic Republic of the Congo, another outbreak was already brewing. 

Congo has said the first person died from the virus on April 24 in Bunia, but it wasn't until May 14 when the first confirmation of Ebola came, by which point tens of people already succumbed to the disease.

On May 17, the WHO declared the Ebola outbreak a public health emergency of international concern. As of June 22, there were 1,094 confirmed cases and 277 confirmed deaths.

With memories of Covid-19 still fresh, how close is the world to another global pandemic, and what could it be caused by this time?

"It is not a matter of if, but when," warns Dr Margaret Harris, public health doctor and lecturer in global public health.

And while it is impossible to predict when the next pandemic might happen, many scientists agree that probability increases due to human activities and their impact on the environment.

"We're seeing more and more instances of the viral families, the bacterial families getting more and more into the human population, probably because we're squeezing them out of their normal environments," says Dr Harris.

"We're making it more and more likely that another microbe, most likely a new virus, will find its way into the human population and wreak the havoc that we did see so much of between 2020 and 2025."

According to data from Metabiota, a data science and epidemiological company focused on quantifying, mitigating, and predicting infectious disease outbreaks, the annual probability of a zoonotic spillover event triggering a pandemic of COVID-19 proportions, or perhaps even greater, is estimated to be between 2.5% and 3.3%.

This suggests a one-in-two likelihood that an outbreak of similar magnitude will emerge over the next 25 years.

When it comes to what could be the cause of the next global pandemic, several types of pathogens are considered plausible candidates.

Public health experts focus especially on organisms that combine several dangerous characteristics: efficient human-to-human transmission, little or no pre-existing immunity in humans, the ability to mutate rapidly, and the potential to cause severe disease or overwhelm healthcare systems.

Here are the major categories scientists and international health agencies monitor most closely.

Influenza viruses still top the list

Influenza viruses remain one of the most serious pandemic threats because they spread more easily, evolve extremely quickly, and circulate widely in birds, pigs, and other animals.

Unlike many viruses, influenza has a segmented genome, which allows different strains to exchange genetic material through a process called reassortment.

This can suddenly create entirely new variants capable of infecting humans.

"The influenzas are usually the ones on top of the list, particularly the avian influenzas, because we've seen a lot of die-off in not just the bird populations but the mammal populations from a number of strains of avian influenza, and we have seen it get into the dairy farms and the herds," notes Dr Harris.

Particular concern surrounds highly pathogenic avian influenza strains such as H5N1, H7N9, and H5N6.

These strains have already infected humans sporadically, often causing severe pneumonia, organ failure, and death.

H5N1 is especially alarming because human cases have shown mortality rates far higher than seasonal influenza.

Fortunately, current strains do not yet spread efficiently between humans.

"It hasn't achieved the ability to spread from human to human rapidly," explains Dr Harris, "but should it do so, it would then be able to spread very widely, very quickly, and cause enormous, enormous havoc."

The deadliest pandemic in modern history, the 1918–1920 so–called 'Spanish flu' pandemic, is estimated to have killed from 17 to 50 million people, and there are several factors that make influenza a likely culprit for the next one.

Firstly, it easily spreads through respiratory droplets and aerosols, and infected individuals can transmit the virus before realizing they are sick.

In combination with global air travel, that enables a rapid international spread such as seen during the COVID-19 pandemic.

Animal reservoirs – species that naturally harbor disease-causing pathogens, often without becoming seriously ill themselves – make the eradication of influenza basically impossible, and with constant mutation, the vaccine effectiveness can reduce over time.

The greatest fear when it comes to the influenza virus is that a bird flu strain could acquire mutations allowing sustained human-to-human transmission while retaining high virulence.

Such a combination could create a pandemic significantly deadlier than COVID-19.

Coronaviruses have proven pandemic capabilities

Coronaviruses became a central pandemic concern after the emergence of SARS, MERS, and COVID-19. These viruses are widespread in bat populations and possess a strong ability to recombine genetically, producing entirely new variants.

Researchers are particularly concerned about novel bat-derived coronaviruses, many of which were responsible for major human outbreaks in the past.

Scientists also worry that repeated human exposure to wildlife habitats increases opportunities for new coronavirus spillovers. Climate change and urban expansion may accelerate these encounters.

But spillover events can also occur at wildlife markets or during intensive livestock farming.

While most of those events either do not lead to anything or result only in isolated cases, the true danger lies in recombination between animal and human coronavirus strains and the pathogen adapting to effectively transmit between humans.

Similarly to influenza viruses, coronaviruses are dangerous because they can spread efficiently through the air and infect people before symptoms appear. They can also cause both mild and severe illness, allowing silent transmission chains, while also being able to mutate in ways that partially evade immunity.

COVID-19 demonstrated how quickly a respiratory coronavirus can overwhelm global healthcare systems and disrupt economies. A future coronavirus with the transmissibility of COVID-19 variant Omicron but the lethality of MERS would represent a catastrophic scenario.

Filoviruses scare with their lethality 

Filoviruses are receiving a lot of coverage these days due to the ongoing Ebola outbreak in the DRK.

These pathogens, including Ebola and Marburg viruses, are among the deadliest viruses known, capable of causing hemorrhagic fever, internal bleeding, multi-organ failure, and shock.

Current Ebola outbreaks are usually contained because transmission requires close contact with bodily fluids.

"It's much easier to contain than a respiratory virus, but it is so lethal and so dangerous to anyone who gets infected that just managing it takes up enormous amounts of resources, and you really need a really strong healthcare system to be able to deal with it," says Dr Harris. 

Scientists fear that even modest increases in transmissibility could dramatically change the global risk profile of these viruses. An Ebola-like virus that spread more efficiently through the respiratory route would pose an unprecedented public health emergency.

Marburg virus has recently drawn increased attention because several outbreaks have occurred in new geographic areas, suggesting broader ecological spread.

Paramyxoviruses: the deadly pathogens waiting for their moment

While perhaps not as widely known as influenza viruses and coronaviruses, the paramyxovirus family includes several highly lethal zoonotic diseases, including Nipah and Hendra viruses.

Nipah virus in particular is considered one of the most dangerous known emerging pathogens, given extremely high human fatality rates and the ability to damage both the respiratory system and brain.

It naturally circulates in fruit bats, which are widespread across Asia and capable of transmitting the virus to livestock and humans.

What is particularly concerning is that the virus possesses the ability for human-to-human transmission, and there are currently no widely available vaccines or specific antiviral treatments. Many infected individuals develop encephalitis, seizures, coma, or severe respiratory distress. 

Some outbreaks have shown evidence of transmission through coughing and close contact, raising fears that the virus could eventually evolve more efficient airborne spread.

At present, outbreaks remain localized and relatively limited. However, should Nipah acquire mutations enabling easier respiratory transmission, it could become one of the deadliest pandemics in modern history due to its combination of high lethality and lack of immunity in humans.

Antimicrobial-resistant bacteria: the pandemic that's already happening

Not all future pandemics will necessarily be caused by viruses.

Drug-resistant bacteria are increasingly viewed as a "slow-motion pandemic" already underway, with major threats including multidrug-resistant tuberculosis (MDR-TB), carbapenem-resistant Enterobacteriaceae (CRE), methicillin-resistant Staphylococcus aureus, and drug-resistant gonorrhea.

These organisms evolve resistance to antibiotics through overuse and misuse of antimicrobial drugs in medicine and agriculture.

Antimicrobial resistance is dangerous for several reasons, including making routine infections untreatable, increasing surgical risks, complicating cancer therapy and organ transplantation, as well as causing hospital outbreaks that are difficult to control.

Tuberculosis, which, despite being a preventable and completely curable disease, claims approximately 1.23 million lives worldwide each year is particularly worrying because it already spreads through the air.

A highly drug-resistant TB strain with broader global spread could produce a prolonged international health crisis.

Unlike explosive viral pandemics, bacterial resistance spreads gradually, but its long-term effects could be equally devastating by undermining modern medicine itself.

Vector-borne viruses could go global with climate change

Mosquito-borne viruses are also increasingly monitored as climate change expands the geographic range of disease-carrying insects. Important examples include dengue virus, zika virus, chikungunya virus, and yellow fever virus.

What is dangerous about these viruses is that as climate change expands their geographic reach, rising global temperatures enable mosquito populations to thrive in previously inhospitable regions, potentially placing billions more at risk.

Dengue alone already infects hundreds of millions annually. A more virulent or globally adapted mosquito-borne virus could strain healthcare systems worldwide, especially in densely populated tropical and subtropical regions.

Zika demonstrated another major concern – some emerging viruses may cause severe birth defects or long-term neurological complications even when symptoms are mild in adults.

Unknown "disease X" keeps scientists awake at night

The World Health Organization uses the term "disease X" to describe a hypothetical, previously unknown pathogen capable of causing a major international epidemic or pandemic.

This concept reflects the possibility that the next pandemic pathogen may be previously undiscovered, either emerging from wildlife reservoirs as environmental disruption and climate change increase the risk of spillover or originating from a laboratory.

In such a scenario, it is plausible that surveillance systems could overlook early signals, leaving humanity to face a threat for which there is no pre-existing immunity, specific antiviral treatments, or available vaccines.

Historically, many major outbreaks were unexpected before they emerged, including HIV/AIDS, SARS, and COVID-19.

"That's what COVID-19 was – completely new – so our bodies didn't have any experience, didn't have any immunological protection from past infections, but also the scientific community was starting from scratch," explains Dr Harris.

The next pandemic will likely come from an animal

Most scientists believe the next pandemic will likely begin with zoonotic spillover, the transmission of a pathogen from animals into humans.

The likelihood of such spillover events is intensified by a range of human-driven factors, including the wildlife trade and live animal markets, intensive livestock farming, deforestation, and the expansion of mining and agriculture, alongside increasing urbanization into previously undisturbed wildlife habitats.

As humans disrupt ecosystems, contact with wildlife reservoirs increases. Bats, rodents, birds, and primates all harbor viruses capable of crossing species barriers.

Once a pathogen adapts to humans, crowded cities and global travel networks can allow rapid international spread within days or weeks.

What makes a pathogen pandemic-capable?

A pathogen becomes especially dangerous on a global scale when it combines several key characteristics, particularly airborne or respiratory transmission, the ability to spread before symptoms appear, and high mutation rates.

These traits allow infections to move silently through populations before health authorities can detect or contain them, making rapid international spread far more likely.

Modern travel and global trade networks further amplify this danger by enabling infected individuals to carry diseases across continents within short periods of time.

Pathogens that mutate quickly may also evade immune responses, reducing the effectiveness of prior immunity and making outbreaks more difficult to control.

This ability to adapt can allow a disease to persist even after public health measures are introduced. The threat becomes even greater when vaccines or effective treatments are unavailable, leaving populations vulnerable during the early stages of an outbreak.

Some pathogens also persist in animal reservoirs, meaning they can continue circulating in wildlife or livestock and repeatedly spill back into humans, making eradication extremely difficult even after major outbreaks are brought under control.

A disease does not need an extremely high fatality rate to become catastrophic. Pathogens with moderate lethality but very high transmissibility often produce the greatest total societal impact because they infect vastly larger numbers of people.

Is the world ready to tackle the next pandemic?

Many experts argue that the greatest pandemic threat is not only the pathogen itself but also the weaknesses within human systems that allow outbreaks to escalate uncontrollably.

Even a dangerous virus can often be contained with rapid action, strong healthcare infrastructure, and effective international cooperation.

However, when governments and institutions respond slowly or inconsistently, diseases can spread far beyond their original point of emergence.

The eighth Global Preparedness Monitoring Board report – titled A World on the Edge – revealed that humanity is at greater risk of a pandemic than a decade ago.

Some of the factors that it highlighted include global vulnerabilities such as delayed political decision-making, poor coordination between countries, and underfunded public health systems, all of which can severely weaken outbreak responses.

But Dr Harris is convinced there are positives to take from the recent events: "The fact that the world reacted very well to the hantavirus outbreak and cooperated well and managed the outbreak on the ship indicates that the lessons from COVID-19 are still fresh."

She points out that the lessons learned from COVID-19 have also been applied in the Pandemic Treaty, showing the world was ready to get together and try to put those things down on paper and make agreements.

"Sadly, parts of that treaty have not been fully negotiated, including something pretty critical, which is sharing of virus samples, [and thus] the benefits that are generated from those virus samples, like tests and treatments," she explains.

"[That] means that the world still hasn't realized that the first rule of fighting a pathogen is working together, not against each other."

Dr Harris also warns that the fact that our attention has gone from health means we are more vulnerable to any future pandemic. 

"We're wasting vast amounts of money on blowing each other up – we're spending enormous amounts of political capital, energy, human resources, economic wherewithal, and infrastructure on things that not only harm us, but it also means we will be in a much weaker position should the pathogen arise.

"And we're also creating situations where it's more likely to happen: you're displacing large numbers of people, you've got higher rates of hunger, you've got more disruption, you've got weaker health systems, and those are all the conditions under which a novel pathogen can arise and take hold in a population and cause massive destruction," she adds.

According to Dr Harris, it is crucial that the global community realize the importance of strong healthcare systems if it hopes to tackle the next pandemic effectively.

"Health isn't a nice-to-have; it's a must-have. It isn't something that's an annoying drain on resources, it's an investment in your protection and the health of your society."