“The thing that would immediately make all of us uneasy is if this person has no direct contact with someone who comes from an affected country,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University.

“That would suggest there are other undetected cases out there, and we have already started some low-grade transmission.”

Public health officials said the infected individual was a resident of Solano County, Calif., and was receiving medical care in Sacramento County, but they have not disclosed any other information to protect the patient’s privacy.

Doctors in the patient’s community may want to expand testing for the coronavirus among their patients in order to identify other infected individuals there, Dr. Schaffner said.

“Physicians are going to want to start testing everybody with a fever and a cough,” he said. “They’re going to want to do that and the patients are going to want to do that.”

Right now, however, testing capacity is limited in the United States.

The C.D.C. developed a diagnostic test and distributed testing kits to local health departments around the country.

But the kits were flawed, and the agency must manufacture new ones. Although a dozen states are capable of testing for coronavirus infection, confirmatory tests must still be done by the C.D.C., a process that can take days.