Editor’s note:

Following the spread of COVID-19 in the US, inappropriate remarks made by American politicians and media outlets have led to a new wave of discrimination against Asian communities, especially Chinese Americans. The Trump administration has been blaming China in order to cover up its failure in curbing the spread of the lethal virus, publicly associating the virus with China, while some media outlets, such as Fox News, have been leading the charge in targeting China, accusing it of “releasing the virus from a bio lab” despite medical professionals and the WHO rejecting such rumors.

The New York Times on April 8 published an article titled “Most New York Coronavirus cases came from Europe,” noting that the virus that has been rampaging through New York was actually brought by European travellers, not Asians, while the virus began to circulate in the New York area by mid-February, weeks before the first confirmed case.

Below is an abbreviation of the original article.

New research indicates that the coronavirus began to circulate in the New York area by mid-February, weeks before the first confirmed case, and that travelers brought in the virus mainly from Europe, not Asia.

“The majority is clearly European,” said Harm van Bakel, a geneticist at Icahn School of Medicine at Mount Sinai, who co-wrote a study awaiting peer review.

A separate team at N.Y.U. Grossman School of Medicine came to strikingly similar conclusions, despite studying a different group of cases. Both teams analyzed genomes from coronaviruses taken from New Yorkers starting in mid-March.

The research revealed a previously hidden spread of the virus that might have been detected if aggressive testing programs had been put in place.

On Jan. 31, President Trump barred foreign nationals from entering the country if they had been in China during the prior two weeks.

It would not be until late February that Italy would begin locking down towns and cities, and March 11 when Mr. Trump said he would block travelers from most European countries. But New Yorkers had already been traveling home with the virus.

“People were just oblivious,” said Adriana Heguy, a member of the N.Y.U. team.

Dr. Heguy and Dr. van Bakel belong to an international guild of viral historians. They ferret out the history of outbreaks by poring over clues embedded in the genetic material of viruses taken from thousands of patients.

Viruses invade a cell and take over its molecular machinery, causing it to make new viruses.

The process is quick and sloppy. As a result, new viruses can gain a new mutation that wasn’t present in their ancestor. If a new virus manages to escape its host and infect other people, its descendants will inherit that mutation.

Tracking viral mutations demands sequencing all the genetic material in a virus — its genome. Once researchers have gathered the genomes from a number of virus samples, they can compare their mutations.

Sophisticated computer programs can then figure out how all of those mutations arose as viruses descended from a common ancestor. If they get enough data, they can make rough estimates about how long ago those ancestors lived. That’s because mutations arise at a roughly regular pace, like a molecular clock.

Maciej Boni of Penn State University and his colleagues recently used this method to see where the coronavirus, designated SARS-CoV-2, came from in the first place. While conspiracy theories might falsely claim the virus was concocted in a lab, the virus’s genome makes clear that it arose in bats.

There are many kinds of coronaviruses, which infect both humans and animals. Dr. Boni and his colleagues found that the genome of the new virus contains a number of mutations in common with strains of coronaviruses that infect bats.

The most closely related coronavirus is in a Chinese horseshoe bat, the researchers found. But the new virus has gained some unique mutations since splitting off from that bat virus decades ago.

Dr. Boni said that ancestral virus probably gave rise to a number of strains that infected horseshoe bats, and perhaps sometimes other animals.

“Very likely there’s a vast unsampled diversity,” he said.

Copying mistakes aren’t the only way for new viruses to arise. Sometimes two kinds of coronaviruses will infect the same cell. Their genetic material gets mixed up in new viruses.

It’s entirely possible, Dr. Boni said, in the past 10 or 20 years, a hybrid virus arose in some horseshoe bat that was well-suited to infect humans, too. Later, that virus somehow managed to cross the species barrier.

“Once in a while, one of these viruses wins the lottery,” he said.

In January, a team of Chinese and Australian researchers published the first genome of the new virus. Since then, researchers around the world have sequenced over 3,000 more. Some are genetically identical to each other, while others carry distinctive mutations.

That’s just a tiny sampling of the full diversity of the virus. As of April 8, there were 1.5 million confirmed cases of Covid-19, and the true total is probably many millions more. But already, the genomes of the virus are revealing previously hidden outlines of its history over the past few months.

As new genomes come to light, researchers upload them to an online database called GISAID. A team of virus evolution experts are analyzing the growing collection of genomes in a project called Nextstrain. They continually update the virus family tree.

The deepest branches of the tree all belong to lineages from China. The Nextstrain team has also used the mutation rate to determine that the virus probably first moved into humans from an animal host in late 2019. On Dec. 31, China announced that doctors in Wuhan were treating dozens of cases of a mysterious new respiratory illness.

In January, as the scope of the catastrophe in China became clear, a few countries started an aggressive testing program. They were able to track the arrival of the virus on their territory and track its spread through their populations.

But the United States fumbled in making its first diagnostic kits and initially limited testing only to people who had come from China and displayed symptoms of Covid-19.

“It was a disaster that we didn’t do testing,” Dr. Heguy said.

A few cases came to light starting at the end of January. But it was easy to dismiss them as rare imports that did not lead to local outbreaks.