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At least 34 people in the United States are infected with the coronavirus spreading from China, federal health officials said on Friday.

“This new virus represents a tremendous public health threat,” Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said at a news briefing.

But so far there has been no community spread of the infection in the United States, she added; all of the cases have been linked to overseas travel.

Eleven of the infections were diagnosed in travelers who fell ill after returning on their own from overseas, and two of their close contacts became infected. The other 21 patients are people who were returned to the United States by the State Department.

More infections are expected among the people who were passengers aboard the Diamond Princess cruise ship and are currently in quarantine, Dr. Messonnier said.

The coronavirus swept through the Diamond Princess after it docked in Yokohama, Japan, and the passengers were not allowed to disembark. More than 600 people onboard have been infected with the virus, testing has shown.

Passengers from the ship account for 18 of the 21 cases in repatriated people in the United States. The other three patients had been evacuated earlier from Wuhan, China, the center of the epidemic.

Because the Diamond Princess passengers may have been in contact with infected people on the ship, “they are considered at high risk for infection,” Dr. Messonnier said.

Ten other passengers from the Diamond Princess tested positive for the virus in Japan, but are not being counted among the infected yet, because the C.D.C. has not confirmed the tests.

They will most likely be added to the case count, Dr. Messonnier said. Several Americans remain critically ill with the infection in Japanese hospitals.

On Thursday, the State Department posted an advisory urging U.S. citizens to “reconsider” traveling by cruise ship to or within Asia. The department had already advised against travel to China.

Of 329 people who were evacuated on two State Department flights, 18 are hospitalized, Dr. Messonnier said: 11 in Nebraska, five near California’s Travis Air Force Base and two near Lackland Air Force Base in San Antonio.

Fourteen of the Americans were found to have positive test results for the virus just before boarding two charter flights back to the United States. The State Department overruled objections from C.D.C. officials and placed the infected on the plane with healthy passengers.

Reports of the disagreement were first published by The Washington Post.

Asked during the briefing about the episode, Dr. Messonnier referred the question to Dr. William Walters, executive for operational medicine at the State Department’s medical services bureau.

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.

Passengers quarantined at Lackland Air Force Base were to be moved on Friday to a different part of the base, to less uncomfortable quarters with more reliable heating and cooling, a C.D.C. official, Dr. Nancy Knight, told them during a conference call on Thursday.

She said all were being offered tests for the virus, but would be required to stay for the full 14-day quarantine even if they tested negative. The reason is that if an infected person is still in the incubation period, the test may not be able to detect the virus yet.

Anyone with a positive test will be tested again and will not be released until two consecutive tests give negative results, Dr. Knight said. The samples have to be sent to Atlanta, so each test takes a few days to complete.

The testing is not mandatory, so people who have no symptoms and decline testing will be free to go after 14 days.

In couples staying in the same room at the base, if one becomes ill, then the other must begin the 14-day quarantine process all over again, starting with the day the partner’s symptoms began.

Because the quarantine is on a military base, all incoming mail has to be inspected, which can delay delivery by several days. One man on the conference call with Dr. Knight said he was expecting prescription medicine by mail and worried that his supply would run out before he would be allowed to receive the new shipment.

Officials on the call did not offer a way to expedite delivery for him.

As of Friday, the coronavirus has infected more than 76,000 people, the vast majority in mainland China, and has killed at least 2,240.