Scientists analysing the SARS-CoV-2 virus say there's no way it could be a bioengineered weapon, as some have suggested.
As of midday Monday, the virus - which causes COVID-19 - had infected at least 330,000 people and killed 14,450.
Early in the virus' spread, there were suggestions it was created in a Chinese laboratory and released, either accidentally or deliberately. Far-right websites and media figures like radio host Rush Limbaugh and former Donald Trump campaign strategist Steve Bannon have pushed the conspiracy theory.
And there have been counterclaims from Chinese officials the US may have deliberately introduced the virus to China.
"It might be US army who brought the epidemic to Wuhan... Be transparent! Make public your data! US owe us an explanation," Chinese Foreign Ministry spokesman Zhao Lijian wrote on Twitter earlier in March.
But a new analysis of the virus has found no evidence for such claims, according to a study published in journal Nature Medicine.
"By comparing the available genome sequence data for known coronavirus strains, we can firmly determine that SARS-CoV-2 originated through natural processes,"said co-author Kristian Andersen, an associate professor of immunology and microbiology at Scripps Research.
Coronaviruses have been around for a long time, and responsible for both 2003's SARS and 2012's MERS (Middle East Respiratory Syndrome) outbreaks.
Chinese scientists quickly sequenced the SARS-CoV-2 virus' genome and made it available for others around the world, to speed up the hunt for a cure and vaccine.
The authors of the latest study focused their attention on the virus' spike proteins, which allow it to grab onto other cells and take them over. In addition to being able to trace the virus back to a single patient zero, the scientists say they found two smoking guns proving it is a natural disease, and not a bioweapon.
Firstly, the spike protein was "so effective at binding the human cells, in fact, that the scientists concluded it was the result of natural selection and not the product of genetic engineering".
Viruses have an incredibly high rate of reproduction, meaning mutations happen quickly - for example, the influenza virus evolves so efficiently we have to develop new vaccines every year, and even then they don't always work. It wouldn't take long for a mildly beneficial mutation to quickly evolve into something better than humans could think up.
The second piece of evidence was the virus' molecular structure, which the scientists compared to a backbone.
"If someone were seeking to engineer a new coronavirus as a pathogen, they would have constructed it from the backbone of a virus known to cause illness," they said.
But instead they found it "differed substantially" from previous known coronaviruses that could infect humans, and "mostly resembled related viruses found in bats and pangolins".
This backs up prior research suggesting the virus made the leap to humans via bats and pangolins.
"These two features of the virus, the mutations in the RBD portion of the spike protein and its distinct backbone, rules out laboratory manipulation as a potential origin for SARS-CoV-2," said Dr Andersen.
This leaves two possibilities for the origin of the virus: either it made the leap to humans and then evolved its deadly capabilities, or it was deadly before it made the jump. The latter makes it far more likely there will be future outbreaks of COVID-19, the scientists said.
"The chances are lower of a non-pathogenic coronavirus entering the human population and then evolving properties similar to SARS-CoV-2," said co-author Andrew Rambaut of the University of Edinburgh.