Dr. Walters said that the test results arrived shortly before the passengers were to board the plane, and that on the flight the infected people were seated separately from the rest. He also said that despite the positive tests, the infected people had no symptoms.

“The decision was the right one in bringing these people home,” he said.

Dr. Messonnier said that decisions were being made “in real time" and that “different perspectives will be brought to the table.” Nonetheless, she added, “We are one U.S. government working together.”

The response to the outbreak is being handled “seamlessly,” she said.

So far, Dr. Messonnier said, there has been no community spread of the disease in the United States, which means no cases in which the source of the infection is not known.

But looking ahead, community spread within the country is very possible and maybe even likely, she said.

Cases detected by local health departments in travelers returning independently from overseas, or in their close contacts, are counted separately from infections in people who were evacuated from China by the State Department.

The reason for the distinction, Dr. Messonnier said, is that the cases found locally more accurately represent what is happening within the United States.

The C.D.C. is still performing most of the testing for the virus, Dr. Messonnier said. The agency had sent diagnostic test kits to all the states, so they could perform their own tests instead of sending samples to Atlanta. But the new kits were flawed and have yet to be replaced.