When the virus hunters landed in the remote African village of Yambuku in 1976, the nuns warned them to stay back. The sisters had cordoned off the area where they were caring for people dying from a mysterious new disease. In fact, the whole village was sealed off, both by thick jungle and now quarantine – the only plane allowed in was the one carrying this special team of international doctors suddenly at their door.

“Stay back or you’ll die!” the nuns yelled.

“It’s OK, we’re here to stop the epidemic,” a young microbiologist named Peter Piot shouted back.

Weeks earlier, blood from a nun who had fallen ill with the mystery disease had arrived at his lab in Belgium, Piot recalls, with a bloodstained note from a local doctor who wondered if the mosquito-borne yellow fever could be to blame. Only this seemed even worse. Whole families succumbed, racked by uncontrollable vomiting and diarrhoea, bleeding from the eyes and mouth. When Piot helped isolate the virus from the sample, he found something entirely new. “It was big [under the] electron microscope, and long like a worm,” he says. It was Ebola.

Piot and the team soon traced its spread to unsterilised needles and the highly infectious blood of its victims. Eventually, science tracked it further back to African fruit bats and grisly outbreaks among monkeys, chimps and gorillas. As more forest was logged, and animals and humans collided, Ebola would break out again and again. But tales of how Piot and his colleagues braved the microbe to contain that first human Ebola cluster would become an epidemiological legend. “That was really the beginning of outbreak investigation,” says Piot, now the director at the London School of Hygiene and Tropical Medicine.

Today, with the world in the grip of its biggest pandemic since the 1918 flu, Bill Gates has called for a crack team of specialists to be embedded in healthcare systems around the globe, ready to contain an outbreak, as Piot and his colleagues once did. Others say we should reduce the risk of pandemics where they emerge – at the edge of the wild – by curbing logging and the wildlife trade from which both SARS and likely COVID arose.

They also say pandemics are more likely than ever. But why? And what are the best ways to stop the next one?

Where do pandemics come from?

We live on a virus planet, says Piot. There are more on Earth than stars in the sky. Fortunately, most of them can’t infect us. The few that can are mostly caught from animals – in that knife-edge moment of chance known as a “spillover” when a pathogen circulating in one species leaps to another. Some animals, such as bats, have evolved for thousands of years alongside their viruses. But as different species interact, the viruses that have grown up with one host have a chance to jump to another.

Most will fizzle out just as suddenly, but sometimes a virus will mutate into something new, even into something that can infect humans. Everything from Ebola and HIV to yellow fever, flu, and SARS began this way. “And humans can give their viruses to animals too,” says evolutionary virologist Professor Eddie Holmes, pointing to outbreaks among farmed pigs. “We’re not the end of the chain by any means.”

The wet market in Wuhan where COVID first emerged in late 2019 was the perfect place for such a spillover – and multiple studies of both the initial outbreak and COVID’s viral ancestry consistently point to it as the most likely origin of the pandemic. Holmes had been to that very market in Wuhan only five years earlier. In the south-western corner, where the fare on offer was often illegal wildlife, he’d photographed a fluffy raccoon dog (a species linked to the SARS outbreak along with civets at a wet market 20 years before).

Such wet markets are still popular throughout Asia, says Nobel-Prize-winning immunologist Professor Peter Doherty – part of an enduring preference for fresh meat over frozen. They are also notorious melting pots of disease, as stressed animals from all corners of the world are caged close together, trading unfamiliar germs, and then handled and butchered by humans. “We worry about it all the time with bird flu too,” says Doherty. “You get wild birds coming into market alongside poultry.”

Not all viruses are primed to go global, of course. “I don’t really worry about an Ebola pandemic because you need close contact [to spread it],” says Piot. “Though we have seen terrible outbreaks like in West Africa in 2014 when 11,000 died.” His eye is still firmly on influenza, which mutates fast and spreads easily. “We haven’t really had flu seasons for the past two winters [during the pandemic]. That means we’re not protected. Something could emerge and rip through.”