This has set off fresh fears about the spread of the disease.
New York Mayor Bill de Blasio said residents were safe to travel around the city, even as officials disclosed that Doctor Craig Spencer had ridden subways, taken a taxi and visited a bowling alley since returning from Guinea on 17 October.
Spencer, 33, had worked with the humanitarian organisation Doctors Without Borders in Africa. He first developed Ebola symptoms on Thursday morning and was taken from his Manhattan apartment to isolation at Bellevue Hospital by a team wearing protective gear, according to city officials.
He had been taking his temperature twice a day since coming home, said city Health Commissioner Mary Travis Bassett.
Two friends and his fiancée are quarantined, she said. His fiancée was quarantined at the hospital, and all three were healthy. The taxi driver did not come into close contact and was not considered at risk, she said.
(READ MORE: “Collateral” death toll expected to soar in Africa’s Ebola crisis)
Spencer is the ninth Ebola case seen in the United States and the first case in America’s largest city, setting off renewed fears about the spread of the virus, which has killed nearly 4,900 people, largely in Liberia, Sierra Leone and Guinea.
The virus is spread through direct contact with bodily fluids from an infected person and is not airborne.
“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.”
Spencer was not feeling sick and would not have been contagious before Thursday morning, the health commissioner said.
“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 stock market reacted to the new Ebola case after Spencer’s test result was announced late on Thursday. S&P futures fell nine points or 0.45 per cent. The dollar slipped against the euro and the US 10-year Treasury rose, lowering its yield to about 2.24 per cent.
The Centers for Disease Control and Prevention (CDC) will confirm the test results within 24 hours, Bassett said.
(READ MORE: WHO warns of “shadow zones” and unreported Ebola cases)
Spencer’s apartment in Manhattan’s Harlem neighbourhood is isolated and sealed off, the health commissioner said.
“I see no reason for the tenants in the apartment building to be concerned,” she said.
Joyce Harrison, who lives in the building across the street, said, “I feel sorry and hope they can nip this in the bud.
“I'll go right on with my daily routine and hope to God it doesn’t come my way,” she said.
The health commissioner said Spencer completed work in Guinea on 12 October and arrived at John F. Kennedy International Airport in New York on 17 October.
Spencer’s Facebook page, which included a photo of him clad in protective gear, said he stopped over in Brussels.
Spencer has specialised 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 LinkedIn profile described her as working in non-profit management and international development with the Hope Program, a career development program for homeless and welfare-dependent adults.
The first person diagnosed with Ebola on US soil flew from Liberia to Texas and died in a Dallas hospital. Two nurses who treated him became infected and one took a commercial flight with a fever, prompting officials in several states to take steps to become better prepared to contain the virus.
(READ MORE: Ebola cases could reach 550,000 to 1.4 mln by late Jan)
The CDC did not name Spencer but said he “participated in the enhanced screening for all returning travellers from these countries” at Kennedy.
The enhanced screening was introduced this month at five major US airports - including Kennedy - for travellers coming from Guinea, Liberia and Sierra Leone.
The doctor “went through multiple layers of screening and did not have a fever or other symptoms of illness,” the CDC added in a statement.