Cases were first reported outside China in Southeast Asia in January. The United States reported on its first case Jan. 21 in the state of Washington. New York City did not report a case until March 1.
Recent disclosures have thrown that timeline on its head. By studying the genus of the virus, we begin to learn that it has been outside China for much longer than expected, and scientists are now sure that the virus had circulated around the globe much earlier.
Here is what we now know about the changing timeline for when the virus first appeared.
Researcher Francois Balloux at the University College London Genetics Institute and his team in the UK may have narrowed the time window.
They studied virus sequences from a global database, and looked at samples from different places and times.
What they found indicates that the virus did not infect people until the end of last year and "excludes any scenario that assumes that the" virus circulated "long before it was identified," the team wrote. Chinese scientists identified it as a new coronavirus on January 7.
But the team also found that people were infected in Europe, the United States and elsewhere for at least weeks, if not months, before the first reported cases in January and February.
US reason zero: Washington. Or was it California?
Washington state was believed to be ground wool for the virus in the United States, and the first report of a death in the United States was Feb. 29, a patient in the Seattle area.
None of the deceased had traveled to China and had no "significant travel history," said Dr. Sara Cody, the county's chief medical officer.
"(This) tells us that we had social transmission far earlier than we had known," Cody said, "and it suggests that the virus was probably introduced and circulated far earlier than we had known."
"The disease spread under the radar," Alessandro Vespignani, director of the Network Science Institute, told CNN.
New evidence from Europe
Doctors in Paris went over old samples from patients who had flu-like symptoms, but did not test positive for that virus, between December 2 and January 16. One, from a 42-year-old man who worked as a fishmonger, tested positive for the corona virus.
This man had not been to China and had no connection to China, scientists said. It suggested that "the disease was already spreading to the French population at the end of December 2019," the team wrote.
Europe's first case was not reported until January, and Italy, the country most affected by the virus there, reported its first two cases on January 31.
Were flu cases and myocardial infarction Covid-19 actually?
Now, authorities around the United States are starting to go back and test to try to identify how early the virus had circulated in their areas, and look at flu-like cases that weren't really flu.
The Centers for Disease Control and Prevention did not begin testing for coronavirus in people with flu-like symptoms, but tested negative for influenza before February 14.
"The reason we want to do this is to see exactly how this virus came to Chicago and the surrounding suburbs, and how it has spread," the county medical examiner's chief, Dr. Ponni Arunkumar, told CNN Wednesday. "It will help public health, it will help Cook County residents understand how this disease came to Cook County."
CNN's Maggie Fox, Omar Jimenez, Jason Hanna, Sarah Moon and Stella Chan contributed to this report.