In Washington State, researchers compared two cases in the hopes of gaining such insight. One case, which last month became the first in the United States, appeared in a patient from whom health officials took a sample on Jan. 19. Another case that surfaced in the region last week probably descended from it, based on an analysis of the virus’s genetic sequence.

The findings suggest that the virus has been spreading in the community for close to six weeks, according to one of the scientists who compared the sequences, Trevor Bedford, an associate professor at the University of Washington.

If that is true, it could mean that 150 to 1,500 people “have either been infected and recovered or currently are infected now,” said Mike Famulare, a researcher at the Institute for Disease Modeling in Bellevue, Wash., who performed the analysis. Those cases, if they exist, have thus far been undetected.