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 Statesand 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.

Only four Ebola cases have been diagnosed so far in the United States: Thomas Eric Duncan, who died on October 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."

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.

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.

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

(Reuters)