The C.D.C. outlined how essential employees can go back to work even if they have been exposed. Scientists warn that the virus might not fade in warm weather. 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 it was brought to the region mainly by travelers 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 — the site of the virus’s first known outbreak — during the previous two weeks. Viruses invade a cell and take over its molecular machinery, causing it to make new viruses. An international guild of viral historians ferrets out the history of outbreaks by poring over clues embedded in the genetic material of viruses taken from thousands of patients.