Somewhere in China, perhaps in the southern Yunnan province, there’s a cave that may hold the mysterious origins of the deadly coronavirus that’s infected thousands, cut off millions of Chinese from their jobs and families and wreaked havoc in global financial markets.

Peter Daszak, a disease ecologist at nonprofit EcoHealth Alliance, would know. He and his team have suited up and ventured into caves all over China and the rest of world in search of bats and the pathogens they carry. “We go into caves,” said Daszak. “We don’t just walk in. We wear a full-body suit: breathing masks, gloves and all the correct equipment.” What he and other scientists around the globe are concluding is that the rapid spread of human settlements in once-remote regions have put people in ever-closer proximity to virus-carrying animals.

More people, meeting more animals, carrying more diseases — a perfect viral melting pot.

As the human population rises, “the number of those spillover events is rising exponentially. It is a direct product of human activity,” Daszak said. And it’s “a simple mathematical certainty” that there will be more outbreaks like the new coronavirus in the future, he said. Total confirmed cases have exploded in recent days to almost 17,400 Monday, with more than 360 deaths. Some disease modeling experts project there are likely 75,000 or more actual cases, as accurate counts from overwhelmed parts of China are impossible to come by.

The U.S. Centers for Disease Control and Prevention estimates that three out of every four emerging infectious diseases in humans first come from animals. Bats contain the highest proportion of mammalian viruses that are likely to infect people in so-called zoonotic infections, according to research published in 2017 by Daszak in the scientific journal Nature.

“I have 90 percent confidence it is a bat-borne virus,” says Linfa Wang, who heads the emerging infectious disease program at Duke-National University of Singapore Medical School. He has been studying bat origins of human viruses for decades and works with a group of researchers sometimes dubbed “the bat pack.”

The CDC has said it’s preparing for the disease to spread more widely in the U.S. and doesn’t expect to stop all cases at the border.

One of his colleagues at the Wuhan Institute of Virology found that the new coronavirus is more than 96 percent genetically identical to a bat virus from the Yunnan province in the southern China, according to results published in the journal Nature on Monday. Zheng-Li Shi, a top coronavirus expert at the Wuhan Institute of Virology, has been studying the bat viruses with Wang and Daszak for more than a decade. She’s leading an emergency science team to respond to the outbreak in Wuhan, according to a Chinese media report.

The Nature study found that the new coronavirus is a distant cousin of SARS, sharing almost 80 percent of its genetic sequence. SARS, or Severe Acute Respiratory Syndrome, is another coronavirus that swept through China and other countries in 2002 and 2003, eventually killing more than 800 people around the globe. It also hijacks the same receptor on lung cells that SARS uses to penetrate cells deep inside the lungs, providing a clue to how it spreads. That receptor is also in the gut, explaining how it may pass through diarrhea as well.

Shi didn’t respond to Bloomberg News inquiries sent via email.

Exactly how the deadly new coronavirus sweeping through China made the leap from animal to human remains a mystery, but scientists say it’s closely linked to urban sprawl and the chaotic and loosely regulated free-for-all of China’s open-air markets. The so-called wet markets, also present in Vietnam, Indonesia and elsewhere around Southeast Asia, feature wild and domesticated animals. They make a perfect mixing ground for viruses.

“These animals are live,” says Christian Walzer, executive director for health at the Wildlife Conservation Society, a New York-based conservation group that also runs zoo and field programs. “You will see a bird on top of a domestic pig, and you might have snake and bats, all stacked together” in wire-mesh cages. Virus-laden fluids and secretions can mix, helping create new viruses, especially when the animals are slaughtered right in front of customers.

“If you planned it and thought, ‘I am going to make new viruses,'” says Walzer, “that is exactly how you would do it.”

The markets may have produced outbreaks in the past that burned out locally. Now, with exploding populations and access to cheap airlines and fast trains, bat viruses from the depths of a jungle can spread to every corner of the globe within days.

In the new coronavirus outbreak, more than 49 of 99 early patients were linked to a market in Wuhan, which also sold wild animals. It now has been closed. The Wildlife Conservation Society is calling for a ban on the markets across Asia. If they aren’t closed, deadly new viruses will emerge every few years, Walzer says.

For years, coronavirus research was considered a backwater. The viruses, named for the crown-like spike on their surface, were known mostly for causing the common cold. The SARS outbreak almost two decades ago abruptly changed things, and helped jump-start a worldwide search for other viruses that could spill into humans after contact with animal excrement, saliva or mucus. The research has often led back to infected bats as the source.

One early clue of bats’ role came from a 1998 outbreak of brain-infecting Nipah virus in Malaysia that killed more than 100 people. It turned out that fruit bats with the virus were feeding on mango trees overhanging a pig enclosure, according to the EcoHealth Alliance. The bats dropped fruit into the pens and infected the pigs; the pigs then passed the pathogen onto people.

To date, researchers have identified at least 200 coronaviruses in bats around the world, according to a recent review in the journal Viruses. In another study, researchers from Columbia University and elsewhere found 12 new coronaviruses in 606 bat samples in Mexico. Due to quirks in their immune system, bats don’t get sick from the myriad viruses they harbor.

Daszak and his colleagues have teams in 10 countries that conduct roughly 50 bat-virus-hunting expeditions a year. In China, trained bat-hunters go into caves to capture the animals, collect oral and genital swabs, as well as urine, feces and blood. The samples are then shipped directly in cold storage to a high-tech lab for testing, Daszak said.

In one crowded bat cave in Yunnan province, Shi and her colleagues from the Wuhan Institute of Virology found coronaviruses containing “all of the building blocks” of SARS. The conditions in the cave were ripe for the viruses to keep mixing, creating the potential for a dangerous new pathogen, they wrote in a 2017 analysis in the scientific journal Plos Pathogens.

“The risk of spillover into people and emergence of a disease similar to SARS is possible,” they said. Several years later, their dire prediction appears to have come true.

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