He predicts either a major war or another pandemic will occur within the next three decades.

Experts in infectious diseases agree with his concerns, if not more so. They say another pandemic isn’t a case of if, it’s now a countdown to when.

The Bill and Melinda Gates Foundation did not respond to Fortune’s request for comment seeking clarification on what the entrepreneur thinks the next pandemic would entail.

But while the Gates Foundation’s work is focused on eradicating polio and diseases caused by unsanitary water, scientists fear that a plethora of diseases present a significant future threat.

This ranges from more aggressive fungal diseases to a new strain of influenza to an outbreak of potentially sexually transmitted diseases.

The good news is that the world health community could take some fairly simple measures to prevent an outbreak as colossal as the COVID-19 pandemic.

The bad news is, they’re not doing it.

What sort of a disease will the next pandemic be?

Professor Paul Hunter of the University of East Anglia highlights that around 75% of emerging infectious diseases are transmitted via random pathogen leaps from animals to humans.

Even decades before the COVID pandemic, Professor Hunter authored papers outlining the need for better enforcement on illegal trading of wildlife.

Animal-human interactions in the trade chain have been blamed by some health authorities for the outbreak of Ebola, SARS, and COVID.

As a result, the next pandemic may originate from a similar source, he added.

Professor Hunter was echoed by Dr Brian Ferguson of the University of Cambridge.

“To give a basic prediction, it probably will be a respiratory disease, purely because they’re the easiest to spread,” explained Dr Ferguson.

“The hardest form of disease to stop is a respiratory-spread disease. We’ve become very good at cleaning water. We haven’t become very good at cleaning air.

“What type of … respiratory infection is very hard to predict. There are lots of options. There’s flu, there’s coronaviruses, there’s other viruses that are spread around respiratory routes [which] could emerge from somewhere in the same way that SARS-CoV-2 did.”

30 years or before?

The general consensus among pathologists, epidemiologists, and health experts is that Gates’s prophecy is likely accurate and may, in fact, prove optimistic.

If Gates is referring to a major, zoonotic (a disease transmitted between species) pathogen pandemic—similar to the scale of COVID—Professor Hunter said it would be impossible to predict a timescale, given how random the leap of diseases between animals and humans can be.

“We could have another [pandemic] next week,” Professor Hunter tells Fortune. “Or we could not have one for another 50 years. Predicting when random events might happen is a nonsense.”

Likewise, Harvard University’s Professor Joseph Allen highlights that a 30-year benchmark might be putting off a problem that could occur sooner: “It’s not a near certainty; it’s a certainty.

“It’s not necessarily wrong to say it could happen within the next 30 years, but I don’t think that we should be [thinking] it’s 30 years away … To put a horizon on it, within the next 30 years? For sure. But we should be clear with people and governments that this doesn’t mean it’s a 30-year-out issue.”

Are we ready for another pandemic?

“Most of my life I’ve considered myself an optimist,” says Professor Hunter. “I’ve grown out of that.”

He’s not alone—many medical professionals are concerned that essential lessons following the COVID pandemic have not been learned.

The concerns raised by the experts Fortune spoke to ranged from global health bodies not working collaboratively enough, vaccines not being shared effectively with poorer nations, a lack of investment when global health isn’t top of the political agenda, and a lack of data about where emerging diseases are originating.

But another issue the scientists touched on is that the public and governments alike are simply exhausted by the possibility of another pandemic.

“I think what Bill Gates was trying to get across … he was talking about things coming that will probably be faster than we’re ready for,” said Dr Ferguson. “It’s something that a lot of people don’t really want to acknowledge because they’ve had enough of COVID.”

Likewise, the psychological damage to health professions across the world after COVID is “profound,” said Professor David Denning of the University of Manchester.

He told Fortune: “If you could imagine being a nurse or a doctor and you were told in a year’s time, or a couple of year’s time, that there’s another pandemic on the way, you’d just want to run. It’s just horrible.”

Supporting an already exhausted medical workforce physically and mentally through another pandemic would be “really, really difficult” he added.

Professor Allen tells Fortune that another simple—and little thought-of—prevention method is adapting society’s built environments to better prevent respiratory diseases.

“If we design buildings for health they can become one of the most important public health interventions of this century—without exaggeration,” he explained.

“COVID showed how short-sighted that was. You had a virus that spread nearly entirely indoors. Our building stock is intentionally designed with low ventilation and poor filtration. Is it any wonder we had the disaster that we had?”

Border controls

A key difference between an epidemic and a pandemic is that the latter can spread viciously across multiple regions, all of which may respond differently to the threat.

Professor Denning says that what stops some diseases—like the Zika virus, for example—is that host species like mosquitos can’t travel huge distances.

A cheering improvement Professor Denning has observed is more thorough testing on imported animals, reducing the risk of infections jumping across borders.

He explained surveillance has improved since COVID, saying: “I was talking to my colleague who works in Canada, and there’s [avian flu in dairy cows] the U.S. now. They’ve managed to prevent it from getting into Canada so far despite masses of trade of cattle across the border. So that’s a very positive thing.

“It’s a mixture of surveillance and appropriate action by veterinarians and public health authorities to prevent transmission.”