It's far more likely that the coronavirus jumped naturally from bats to people than leaked from a lab — at least 4 other viruses did the same thing in the last 45 years

• A fringe theory suggests the new coronavirus could have leaked from a Chinese research laboratory, but there's little evidence to support it.
• It's far more likely that the virus spilled over naturally from bats, jumping to humans via an intermediary animal host.
• This isn't the first instance of a cross-species spillover event, and experts don't think it will be the last.
• Ebola, SARS, Middle Eastern Respiratory Syndrome, and Nipah are all zoonotic diseases that naturally spilled over in the past.
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When Chinese authorities first reported an outbreak of a new coronavirus, many disease scientists had the same thought: History was repeating itself.

The COVID-19 pandemic is the latest example of a well established phenomenon — viruses commonly jump from an animal host to humans. This type of cross-species hop, called a spillover event, also led to outbreaks of Ebola and severe acute respiratory syndrome (SARS). Both of those viruses originated in bats, and genetic research has all but confirmed the same for the new coronavirus — a study published in February found that it shares 96% of its genetic code with coronaviruses circulating in Chinese bat populations.

However, lingering questions about exactly when and how the coronavirus started spreading in people have given rise to a range of unsubstantiated theories. One suggests it may have accidentally leaked from a Wuhan laboratory in which scientists were researching coronaviruses.

While it's not impossible that a stored sample of a bat coronavirus could have leaked from a lab, it's far more likely that it jumped naturally from bats to an intermediary species — perhaps a pangolin — before making its way into our population.

Infectious-disease researchers had been warning of the potential for exactly such a spillover event for years.

Many spillovers have occurred in the past

Experts have long agreed that the next pandemic was always a matter of when, not if.

"The world needs to prepare for pandemics in the same serious way it prepares for war," Bill Gates said during a 2018 conference. "If history has taught us anything, it's that there will be another deadly global pandemic."

The year prior, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, warned of the inevitability of a "surprise outbreak" of a new disease.

"There is no question that there will be a challenge to the coming administration in the arena of infectious diseases," Fauci said during a speech at Georgetown University, specifically highlighting the threat of unknown diseases.

"The thing we're extraordinarily confident about is that we're going to see this in the next few years," he added.

Three out of every four emerging infectious diseases come to us from other species; these pathogens are known as zoonotic diseases.

The 2009-2010 H1N1 pandemic — swine flu — started in pigs then killed nearly 300,000 people. People have caught bird flus via direct contact with infected poultry. Other pandemic influenza strains, including the 1957 "Asian flu" and the 1968 Hong Kong pandemic, likely started in birds, too.

And in the last 45 years, at least four epidemics — including SARS — have been traced back to bats.

"Bats and birds are considered reservoir species for viruses with pandemic potential," Bart Haagmans, a virologist at the Erasmus Medical Center in Rotterdam, Netherlands, previously told Business Insider.

Researchers traced the SARS coronavirus to a population of horseshoe bats in China's Yunnan province; humans caught it from civet cats at a wet market in Guangdong. The virus killed 774 people and infected more than 8,000 in eight months.

Middle East respiratory syndrome (MERS), also a coronavirus, first passed from bats to dromedary camels. It circulated in the camel population undetected for decades before jumping to humans in 2012. So far, 858 people have died across 28 countries from the illness, which is marked by a fever, cough, and shortness of breath.

In Southeast Asia, fruit bats were the original hosts of the deadly Nipah virus, which emerged in Malaysia in 1998, then again in India in 2001. The bats passed it to farmed pigs, which gave it to people. Patients experienced headaches and vomiting; many slipped into a coma and died.

Fruit bats in Africa have played a major role in Ebola outbreaks as well. The worst Ebola outbreak in history, however, came from a population of long-fingered bats. More than 11,000 people died in that epidemic from 2013 to 2016.

Bats are prolific virus carriers

Scientists estimate that there are 1,400 living bat species.

"The great diversity of bats also means they host a great diversity of viruses," Nancy Simmons, curator of the American Museum of Natural History's mammology department, said during a media briefing on Wednesday.

Bats harbor a significantly higher proportion of viruses than other mammals, according to a 2017 study. A 2019 study, in fact, found that bats carry more than 200 coronaviruses. They're ideal hosts if you're a virus because bats fly across large geographical ranges, transporting diseases as they go without getting sick themselves.

Bats pass these virus to other species via their poop or saliva, and the unwitting intermediaries can transmit the virus to humans in the same way.

A study published in March 2019 predicted that bats would be the source of a new coronavirus outbreak in China.

"It is highly likely that future SARS- or MERS-like coronavirus outbreaks will originate from bats, and there is an increased probability that this will occur in China," the researchers wrote at the time.

That's because the majority of coronaviruses — those that affect humans and animals — can be found in China, and many bats "live near humans in China, potentially transmitting viruses to humans and livestock," the authors said.

Millions of people are exposed to zoonotic viruses annually

Peter Daszak, president of EcoHealth Alliance, told NPR this week that his colleagues are "finding 1 to 7 million people exposed" to zoonotic viruses in Southeast Asia each year.

"That's the pathway. It's just so obvious to all of us working in the field," he said.

Dennis Carroll, former director of US AID's emerging threats division, told Nautilus Magazine in March that research from EcoHealth Alliance suggests "we're looking at an elevation of spillover events two to three times more than what we saw 40 years earlier."

That's because of the growing human population and the way we're encroaching on wild areas.

"The single biggest predictor of spillover events is land-use change — more land going to agriculture and more specifically to livestock production," Carroll said.

He added, "whatever future threats we're going to face already exist; they are currently circulating in wildlife."