Coronavirus or influenza? Bacteria or fungi? Experts share where the next pandemic could come from.

Nearly four years after COVID-19 emerged, public health experts already say it’s not a matter of if we’ll have another pandemic — but when. So Yahoo Life asked several experts for their take on where the next pandemic could come from. Here’s what they said.

Viruses

Experts point to a virus as the most likely source of the next pandemic — and for good reason: Most modern pandemics have come from viruses, and there are plenty of types of viruses that could be responsible for the next one.

“My sense is that the next pandemic will also be a viral pathogen transmitted by the respiratory route (rather than fecal/oral or by contact), mainly since the airborne/respiratory route is more efficient,” Dr. Dean Winslow, a professor of infectious diseases at Stanford University School of Medicine says in an email to Yahoo Life. “Either a coronavirus, influenza or parainfluenza virus variants would be good bets.”

A likely reservoir for viruses with pandemic-level potential is animals, and as humans encroach further on animals’s habitats through deforestation, there will be more opportunities for animal viruses to adapt to human hosts.

We’ve already seen plenty of examples of viruses making the jump from animals to humans: Middle East respiratory syndrome (MERS) originated in camels; severe acute respiratory syndrome (SARS) originated in small mammals; HIV originated in chimpanzees; and one COVID-19 theory suggests the disease may have originated in racoon dogs.

“Influenza viruses use birds as their natural reservoir, and certain strains have become adapted to transmission and infection of mammals (especially pigs and humans),” Winslow says. “Similarly, coronaviruses have been known to primarily use bats as their primary reservoir, but with SARS-CoV-2 we saw extensive transmission between other species including humans, deer and both wild and domestic cats.”

Some experts say the next pandemic is most likely to emerge from a virus originating in bats — which have the highest proportion of zoonotic viruses among mammals — rodents or birds.

But what kind of virus could it be? Experts say there are a few probable culprits, including:

• Influenza viruses: You’re likely familiar with the seasonal flu, but in the last century there have also been four influenza pandemics: the infamous Spanish Flu pandemic in 1918, the H2N2 flu pandemic in 1957, the H3N2 flu pandemic in 1968 and the H1N1 flu pandemic 2009. Dr. Allen Cheng, director of infectious diseases at Monash Health, writes that influenza isn’t as infectious as other respiratory infections, but its short incubation period means outbreaks can spread quickly. Dr. Wafaa El-Sadr, an epidemiology professor at Columbia University Mailman School of Public Health, tells Yahoo Life: “There’s concern about influenza and whether a new strain of influenza could arise that could be deadly and could create an outbreak around the world, if not a pandemic.”

• Coronaviruses: These respiratory viruses are responsible for the COVID-19 pandemic as well as other diseases including MERS, which was first reported in 2012, and SARS, which caused a global outbreak in 2003.

• New viruses: “There are some viruses that we haven’t even detected or that we don’t know have a pandemic potential,” Pablo Penaloza-MacMaster, an assistant professor of microbiology and immunology at Northwestern University Feinberg School of Medicine, tells Yahoo Life. “So we have to keep an open mind.” El-Sadr explains that “new viruses” could also mean pathogens that already exist in animals but haven’t been identified in humans before.

Bacteria

Cholera (caused by Vibrio cholerae bacteria) and bubonic plague, or the Black Death (caused by the bacterium Yersinia pestis) are some of the most famous examples of how devastating a pandemic can be. But experts say that today we’re unlikely to see pandemics on that scale caused by bacteria.

“If you go back to the 1300s, we did not have very good infrastructure and very good sanitation measures,” Penaloza says. “Right now, assuming that you have a population with access to clean water and there’s good hygienic measures, then you raise the bar. You have to have a pathogen that has the ability of transmitting in a setting where people wash their hands and have proper sanitation measures — and I think viruses are more likely to cross that threshold than other pathogens [like bacteria].”

El-Sadr says that, in general, “most people would believe that it’s likely to be a virus, rather than a bacterium, but at the same time it doesn’t mean that it’s impossible that a bacterium will be the cause of a pandemic. We're seeing [antibiotic-resistant] bacteria around the world, in almost every country around the world. And that’s causing a lot of disease and mortality as well.”

Antimicrobial resistance — or when germs like bacteria and fungi develop the ability to defeat the drugs designed to kill them — is considered “an urgent global public health threat,” with at least 1.27 million deaths attributed to antimicrobial resistance in 2019.

But El-Sadr points out that right now antimicrobial resistance is mostly restricted to hospital and health care facility settings.

Fungi

Dr. Andrej Spec, who specializes in fungal infections as associate director of the infectious disease clinical research unit at Washington University School of Medicine in St. Louis, previously told Yahoo News that fungi pose a major health threat but are chronically misunderstood.

“It’s quite likely that we’ll have an emergence of a new fungus that may, in the next 20 years, be responsible for a couple hundred thousand deaths a year,” Spec said.

“And you know, we already have many fungi that cause a couple 100,000 deaths a year — we just ignore them, because we have this narrative that fungi are rare,” he added.

The World Health Organization released its first ever list of fungal priority pathogens last year in response to the increased threat of invasive fungal disease. Candida auris, for example, was first discovered in humans in 2009, and since then hundreds of thousands of cases have been identified in countries around the world, including in the U.S. It’s highly drug-resistant and fatal in one-third of patients, and was likely able to adapt to infect humans thanks to rising temperatures due to global warming.

But Spec thinks it’s unlikely we’ll see a fungal pandemic at the scale of the COVID-19 pandemic, where a single virus was responsible for millions of deaths worldwide.

“The fact is, it hasn’t happened ever, so it's very unlikely to actually happen,” he says.

Penaloza says that a biological advantage viruses and bacteria have over fungi is that they replicate much faster — enabling them to spread more easily.

“It depends on the virus, of course, but with a virus you can have millions of copies in one day. With fungi, it doesn’t replicate at those high levels, and the mutation rate of fungi is not as high as viruses,” Penaloza explains.

That said, “It could still happen,” he says. “We shouldn’t rule it out, but I think statistically speaking it’s more likely that [the next pandemic] is going to be a virus. Maybe bacteria, but I think my bet is on viruses more.”