How and why did the COVID-19 pandemic start is an important question for medical researchers. While some assert that the coronavirus that caused the COVID-19 pandemic was leaked, accidentally or deliberately, from China's Wuhan Institute of Virology, most scientists believe spillover from the bats who naturally host these coronaviruses to other animals was the likely cause.

Two strains of the coronavirus have resulted in significant public health infections. SARS-CoV-1 caused an outbreak in 2002-2004 that killed hundreds of people; SARS-CoV-2 was responsible for the worldwide COVID-19 pandemic in 2019-2022.

When different viruses infect the same bat, eventually a new viral combination is sometimes produced. As was the case with COVID-19, the combination can become especially dangerous for humans because few, if any, people have natural immunity against a new virus.

The extent and seriousness of the COVID-19 pandemic that first appeared in Wuhan, China's eighth largest city with a population of over 10 million, nevertheless raised suspicions about non-medical causes. Though the research on coronaviruses in bats at the Wuhan Institute had been partially supported by the U.S. National Institutes of Health (NIH) and other distinguished researchers outside China for many years, some alleged that the pandemic was started by a deliberate lab leak. In April 2020, in the midst of the pandemic, President Trump cancelled the NIH financial support.

With the origin of the pandemic still uncertain, a team of medical scientists decided to study if the particular coronavirus jumped from bats to humans to cause the pandemic.

It turns out the conspiracy theories were likely wrong. First, no one has shown that SARS-CoV-2 was ever at the Wuhan Institute before the pandemic began. Second, this new study, which traced the genetic history of SARS-CoV-2, strongly suggests that this coronavirus responsible for COVID-19 and a different coronavirus twenty years earlier, SARS-CoV-1, which caused severe acute respiratory syndrome (SARS) had followed very similar paths in the real world.

In both cases, the transmission of each coronavirus began with bats, specifically horseshoe bats.

Horseshoe bats have been the natural host for thousands of years for this large family of RNA viruses, coronaviruses, that include the viruses causing SARS and COVID-19. The bats and the viruses they carry live mostly in Western China and Northern Laos, about 2700 kilometers away from Wuhan, where the first human cases of COVID-19 human infection emerged.

Horseshoe bats forage at night over a range of only about 2 to 3 km or about 1.5 to 1.8 miles from their roost. At that rate, it would take several years before these bats and the viruses they carry would have reached far-away Wuhan.

More likely than direct bat transmission to a vulnerable human, the study concluded, the viruses probably jumped first to other animals who were traded in the wild animal marketplace.

The scientists also identified the likely animal culprits — palm civets and raccoon dogs, — both commonly traded for their fur and meat along this long Chinese route to Wuhan.

The virus causing COVID-19 didn't emerge naturally with bat migration or come from a lab leak, but probably developed and was released from wild animals traded for their fur and meat.

These two wild animal species were found to be carrying the bat-hosted coronavirus that caused the first SARS outbreak in humans at the beginning of this century. So the scientists offered an educated guess that the same thing happened with COVID-19 — the coronavirus causing COVID-19 didn't emerge naturally with bat migration or come from a lab leak, but probably was released in the wild animal marketplace.

The question that remains is if these bat-hosted RNA viruses don't kill the bats, why are they so contagious and deadly for human beings and what can we do about it to prevent a new pandemic from a different coronavirus? What happened along this 2700 km route from Central China to Wuhan may guide our prevention planning.

“When two different viruses infect the same bat, sometimes what comes out of that bat is an amalgam of different pieces of both viruses,” said co-senior author Joel Wertheim, a professor of medicine at UC San Diego School of Medicine's Division of Infectious Diseases and Global Public Health, in a statement. After many similar exchanges, eventually a new viral combination is produced, as with COVID-19, that is especially dangerous for humans because few, if any, people have natural immunity against the new virus.

The study researchers examined the genetic fingerprints of members of the coronavirus family and confirmed that SARS-CoV-2, the virus causing COVID-19, emerged along this route from Southeast Asia to Wuhan.

Lessening the risk from the next SARS-like virus that goes through mutations, jumps to other animals and causes another pandemic with a newly arranged virus remains a challenge.

“We show that the original SARS-CoV-1 was circulating in Western China — just one to two years before the emergence of SARS in Guangdong Province, South Central China, and SARS-CoV-2 in Western China or Northern Laos — just five to seven years before the emergence of COVID-19 in Wuhan,” said Jonathan E. Pekar, Ph.D., a 2023 graduate of the Bioinformatics and Systems Biology program at UC San Diego School of Medicine, now a postdoctoral researcher at the University of Edinburgh.

Lessening the risk from the next SARS-like virus that is hosted by bats, goes through mutations, jumps to other animals and causes another pandemic with a newly arranged virus remains a challenge.

Surveillance is key. The Wuhan lab helped American health officials monitor the ongoing threat of new emerging diseases from Asia. It functioned as part of an early warning network. Scientists there regularly take blood samples from bats and are especially on alert for new viral configurations. Because of increased wild animal trading, urbanization and habitat destruction the threat is rising.

It should be noted that while the research establishes a plausible mode of transmission to humans, it does not do so definitively. It is possible that somehow the SARS-CoV-2 virus that caused the pandemic came from a lab rather than naturally through genetic mixing.

Meanwhile, pursuing that possibility has become more difficult. Gain-of-function research by the Wuhan lab and other virology labs is designed to investigate how viruses become more powerful. In a recent May 2025 executive order, President Trump specifically banned U.S. financial support for gain-of-function researchers in China and Iran where the bats live. Without U.S. support, it may become more difficult to catch a bat-hosted coronavirus before it mutates and causes a new disease that infects many Americans.

The study is published in Cell.