Dr. Stephen Kornfeld, the American oncologist and cruise ship passenger who tested positive for the hantavirus, has been moved out of isolation and into a quarantine unit with 15 other passengers being monitored for signs of illness.

Dr. Kornfeld had been placed in a biocontainment unit after testing “mildly positive” for the virus. But he tested negative for the virus after returning to the United States, according to an official who disclosed the information on condition of anonymity. He was “medically cleared” to move to quarantine on Wednesday, according to Nebraska Medicine, the health system overseeing the units.

Health officials have not named Dr. Kornfeld, but he has identified himself as the passenger in isolation in interviews with news media.

He did not have symptoms, but was being monitored along with other U.S. passengers from the MV Hondius cruise ship at special facilities in Omaha, doctors said on Monday. They were all on a cruise from Argentina to the Canary Islands when some of their fellow passengers fell ill with the hantavirus; three died.

Dr. Kornfeld was tested twice before returning to the United States, and only one of those tests had indicated hantavirus, Capt. Brendan Jackson, a top U.S. health official, said on Monday.

Eighteen Americans exposed to the virus were flown back from the Canary Islands on a government flight on Monday, and 15 of them were being monitored in the National Quarantine Unit at the University of Nebraska Medical Center in Omaha, which is designed for people who have been exposed to serious infectious disease.

Two other Americans on that flight, one passenger who was showing symptoms and the person’s partner, were taken to a biocontainment unit at Emory University Hospital in Atlanta, according to Captain Jackson, the acting director of the division of high-consequence pathogens and pathology at the U.S. Centers for Disease Control and Prevention.

That symptomatic person also tested negative for the hantavirus, federal health officials said on Tuesday afternoon.

Hantavirus, a rare family of pathogens carried by rodents, can be deadly, and there is no vaccine or cure. The World Health Organization has identified the Andes subtype of the virus, which is the only one that can spread among people, as the one that affected the cruise ship passengers.

The cruise departed Argentina in April for the Canary Islands. Several other passengers from other countries have become ill or tested positive, health officials have said.