A doctor who worked in West Africa with Ebola patients was in an isolation unit in New York on Friday after testing positive for the deadly virus, becoming the fourth person diagnosed with the disease in the United States and the first in its largest city.

The worst Ebola outbreak on record has killed at least 4,900 people and perhaps as many as 15,000, mostly in Liberia, Sierra Leone and Guinea, according to World Health Organization figures.

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Only four Ebola cases have been diagnosed so far in the United States: Thomas Eric Duncan, who died on Oct. 8 at Texas Health Presbyterian Hospital in Dallas, two nurses who treated him there and the latest case, Dr. Craig Spencer.

Spencer, 33, who worked for Doctors Without Borders, was taken to Bellevue Hospital on Thursday, six days after returning from Guinea, renewing public jitters about transmission of the disease in the United States and rattling financial markets.

Three people who had close contact with Spencer were quarantined for observation – one of them, his fiancée, at the same hospital – but all were still healthy, officials said.

Mayor Bill de Blasio and Governor Andrew Cuomo sought to reassure New Yorkers they were safe, even though Spencer had ridden subways, taken a taxi and visited a bowling alley between his return from Guinea and the onset of his symptoms.

“There is no reason for New Yorkers to be alarmed,” de Blasio said at a news conference at Bellevue. “Being on the same subway car or living near someone with Ebola does not in itself put someone at risk.”

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Health officials emphasized that the virus is not airborne but is spread only through direct contact with bodily fluids from an infected person who is showing symptoms.

After taking his own temperature twice daily since his return, Spencer reported running a fever and experiencing gastrointestinal symptoms for the first time early on Thursday. He was then taken from his Manhattan apartment to Bellevue by a special team wearing protective gear, city officials said.

He was not feeling sick and would not have been contagious before Thursday morning, city Health Commissioner Mary Travis Bassett said.

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Owners of the bowling alley he visited said they had voluntarily closed the establishment for the day as a precaution. But the driver of the ride-sharing taxi Spencer took was not considered to be at risk, and officials insisted the three subway lines he rode before falling ill remained safe.

“We consider that it is extremely unlikely, the probability being close to nil, that there would be any problem related to his taking the subway system,” Bassett said.

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The U.S. Centers for Disease Control and Prevention (CDC) will confirm Spencer’s test results within 24 hours, she said.

Residents, investors rattled

His case brings to nine the total number of people treated for the disease in U.S. hospitals since August, but just two – Duncan’s nurses – contracted the virus in the United States.

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The New York case surfaced days after dozens of people who were exposed to Duncan emerged from the 21-day incubation period with clean bills of health, easing a national sense of crisis that took hold when his nurses, Nina Pham and Amber Vinson, became infected.

“I’m really concerned,” said Kiki Howard, 26, a student who lives on the block next to Spencer’s home in Harlem. “There’s a school at the end of the block. My main concern is for the safety of the children.”

The health commissioner said Spencer’s apartment was isolated and sealed off, noting, “I see no reason for the tenants in the apartment building to be concerned.”

Still, there were signs that the latest Ebola case had unnerved investors. S&P futures fell 9 points or 0.45 percent. The dollar slipped against the euro and the U.S. 10-year Treasury rose, lowering its yield to about 2.24 percent.

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The city health commissioner said Spencer completed work in Guinea on Oct. 12 and arrived at John F. Kennedy International Airport in New York on Oct. 17. His Facebook page, which included a photo of him clad in protective gear, said he stopped over in Brussels.

Spencer has specialized in international emergency medicine at Columbia University-New York Presbyterian Hospital in New York City since 2011.

Columbia, in a statement, said he has not been to work nor seen any patients since his return.

A woman named Morgan Dixon was identified on Spencer’s Facebook page as his fiancée. Her Linked-In profile said she worked in nonprofit management and international development with the Hope Program, a career development agency for homeless and welfare-dependent adults.

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The CDC did not name Spencer but said he “participated in the enhanced screening” instituted for all travelers returning from Guinea, Liberia and Sierra Leone this month at five major U.S. airports – including Kennedy.

The doctor “went through multiple layers of screening and did not have a fever or other symptoms of illness”, the CDC said in a statement.

(Reporting by Ellen Wulfhorst and Sebastien Malo; Additional reporting by Natasja Sheriff, Barbara Goldberg, Jonathan Allen and Laila Kearney in New York; Writing by Steve Gorman; Editing by Louise Ireland)