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.