SHANGHAI - As a team from the World Health Organization works in China to investigate the origins of COVID-19, the following factbox looks at what we know about how the pandemic began.
The coronavirus that causes COVID-19 - known as SARS-CoV-2 - was first identified in the central Chinese city of Wuhan in January 2020, and most scientists believe it is still most likely that it originated in China.
Peter Ben Embarek, WHO expert on food safety and zoonotic diseases, said Wuhan offered "the first solid clues" about the transmission of COVID-19 and any investigation would start there.
Though some studies have suggested COVID-19 was present in Italy or Spain earlier in 2019, and that it might have been responsible for a spike in pneumonia cases in France, few researchers believe it could have entered Wuhan from Europe.
Its closest relative in nature is the RaTG13 virus, which was discovered in horseshoe bats in southwest China's Yunnan province. The 96.2% genetic match between the two makes it highly likely that COVID-19 also originated in the bat colonies of China's southwest border regions.
The initial cluster of infections was traced back to the Huanan seafood market in Wuhan, leading many to assume that "patient zero" was probably a trader exposed to contaminated meat products.
But this version of events may be too simplistic to explain the patterns of infection in Wuhan and elsewhere.
Many of the earliest reported cases had no connection with the market. On Dec. 10, 2019, 41 Wuhan residents were hospitalized with what turned out to be COVID-19, but 13 of them had no link to Huanan.
A team of Chinese researchers said there were two types of SARS-CoV-2 circulating in Wuhan, only one of which was associated with the market.
Scientists also said the virus was unusually "pre-adapted" for rapid human transmission, making it unlikely that the first human contact was made at the seafood market.
WHO investigators are keen to find what the intermediary species was that allowed SARS-CoV-2 to move from its original bat host into humans.
Preliminary scientific papers in China identified snakes and mink as potential candidates, and similar coronavirus infections were also found in pangolins illegally trafficked into China.
But some scholars now believe there was no intermediary species at all, and that a SARS-CoV-2 like virus was transmitted directly from bats to humans, possibly on multiple occasions.
The first people infected were likely to be traders in bat meat, or in bat droppings used in traditional Chinese medicine, and one of them could have carried it into the Huanan seafood market, causing the superspreader event that allowed the pandemic to begin.
"GAIN OF FUNCTION"
Though there is no credible supporting evidence, some researchers still do not rule out the possibility that the virus was released accidentally by a specialist lab at the Wuhan Institute of Virology.
There is no indication that SARS-Cov-2 contains any synthetic insertions, but some researchers say it could have been subject to a process known as "gain of function", where the lab forces the virus to become more infectious by exposing it to human receptor cells.
Some experts say the WHO investigators needs to be granted access to all the research conducted in the lab to see whether there are any closer matches than the RaTG13 virus.
But there is another explanation why the coronavirus has proven to be so lethal.
If SARS-CoV-1 had been circulating in humans throughout southwest China for months before it was finally identified in Wuhan, natural selection itself could have trained it to bind more effectively to those receptor cells.
Fully optimized after months of human exposure, it made an explosive breakthrough at the Huanan market, where conditions were ideal for rapid viral transmission - though it was also very likely spreading elsewhere.