Different types of coronavirus are being spread around the world, experts have discovered.
Covid-19 is believed to have mutated into three distinct strains - Type A, B and C.The original one - Type A - that was identified in Wuhan, China, is now thought to be sweeping the US, with New York now having more cases (160,000) than any country.
England and Europe is believed to be battling Type C.
Researchers from Cambridge University reconstructed the early path of Covid-19 as infection spread from Wuhan, China, in December out to Europe and North America.
It shows that type A - the original virus most closely related to the virus found in both bats and pangolins - was not China's most common. Instead Type A is now prevalent in Australia and the US.
China was mainly hit by Type B, which was in circulation as far back as Christmas Eve. It is believed this type mutates slowly through China, but rapidly outside China.
Type B is derived from A, separated by two mutations, then C is in turn a "daughter" of B, the study suggests.
Type C may have spread to Europe via Singapore.
The findings are published in the journal Proceedings of the National Academy of Sciences (PNAS).
Dr Peter Forster, geneticist and lead author from the University of Cambridge, said: "There are too many rapid mutations to neatly trace a Covid-19 family tree.
"These techniques are mostly known for mapping the movements of prehistoric human populations through DNA.
"We think this is one of the first times they have been used to trace the infection routes of a coronavirus like Covid-19."
Experts believe the virus, called SARS-CoV-2, is mutating to overcome immune system resistance in different populations.
Researchers found that the closest type of coronavirus to the one discovered in bats - type A, the original human virus genome - was present in Wuhan, but was not the city's predominant virus type.
Mutated versions of A were seen in Americans reported to have lived in Wuhan, and a large number of A-type viruses were found in patients from the US and Australia.