Researchers estimate that by March 1, the virus had already infected about 1,000 to 10,000 people who have not yet been accounted for. At the start of this month, about 80 U.S. cases had been confirmed and officials were still expressing confidence they could contain the new virus.

Quarantines, contact tracing and other public health measures have likely tamped down the COVID-19 outbreak here, the researchers said. But from the start, a group of infected travelers just big enough to fill an elevator likely has been expanding the virus’ reach, largely undetected.

Released into a country of about 330 million, each of these travelers was assumed to have passed the virus to 2 to 2.5 people, each of whom in turn infected another 2 to 2.5 people, and so on. Tote up the nodes on this rapidly branching network of contacts and the number of victims balloons quickly, the researchers wrote.