It was announced on Friday probably was descended from that first case.
The two people live in the same county, but are not known to have had contact with one another, and the second case occurred well after the first would no longer be expected to be contagious. So the genetic findings suggest that the virus has been spreading through other people 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 Fred Hutchinson Cancer Research Center and the University of Washington.
Dr. Bedford said it was possible that the two cases could be unrelated, and had been introduced separately into the United States. But he said that was unlikely, however, because in both cases the virus contained a genetic variation that appears to be rare — it was found in only two of the 59 samples whose sequences have been shared from China, where the virus originated.