As vaccines roll out against COVID-19, many experts of emerging infectious diseases are already focused on preventing the next pandemic.

They fear another virus will leap from wildlife into humans, one that is far more lethal but spreads as easily as SARS-CoV-2, the strain of coronavirus that causes COVID-19. A virus like that could change the trajectory of life on the planet, experts say.

"What keeps me up at night is that another coronavirus like MERS, which has a much, much higher mortality rate, becomes as transmissible as COVID," said Christian Walzer, executive director of health at the Wildlife Conservation Society. "The logistics and the psychological trauma of that would be unbearable."

SARS-CoV-2 has an average mortality rate of less than 1%, while the mortality rate for Middle East respiratory syndrome, or MERS — which spread from camels into humans — is 35%. Other viruses that have leapt the species barrier to humans, such as bat-borne Nipah, have a mortality rate as high as 75%.

"There is a huge diversity of viruses in nature, and there is the possibility that one has the Goldilocks characteristics of pre-symptomatic transmission with a high fatality rate," said Raina Plowright, a virus researcher at the Bozeman Disease Ecology Lab. "It would change civilization."

That's why in November the German Federal Foreign Office and the Wildlife Conservation Society held a virtual conference called One Planet, One Health, One Future, aimed at heading off the next pandemic by helping world leaders understand that killer viruses are unleashed on the world by the destruction of nature.

With the world's attention gripped by the coronavirus, infectious disease experts are redoubling their efforts to show the robust connection between the health of nature, wildlife and humans. It is a concept known as One Health. Experts predict it would cost about $700 billion to institute these and other measures, according to the Wildlife Conservation Society. On the other hand, it's estimated that COVID-19 has cost $26 trillion in economic damage.

Infectious disease experts estimate there are 1.67 million viruses in nature; only about 4,000 have been identified. "We've penetrated deeper into eco-zones we've not occupied before," said Dennis Carroll, an emerging infectious disease expert who is setting up the Global Virome Project to catalog viruses in wildlife.

When these things happened a century ago, he said, the person who contracted the disease likely died there. "Now an infected person can be on a plane to Paris or New York before they know they have it," he said.

Researchers say the clock is ticking. "We have high human population densities, high livestock densities, high rates of deforestation — and these things are bringing bats and people into closer contact," Plowright said. "We are rolling the dice faster and faster and more and more often. It's really quite simple.

Kaiser Health News is a national health policy news service. It is an editorially independent program of the Henry J. Kaiser Family Foundation and not affiliated with Kaiser Permanente.