Mayor Bill de Blasio and Governor Andrew Cuomo speak at a press conference in New York City on Thursday. Dr Craig Spencer, who had returned to New York City from Guinea, has contracted the Ebola virus. 

Reuters/New York

A physician with Doctors Without Borders who returned to New York City after treating Ebola victims in West Africa tested positive for the virus, setting off fresh fears about the spread of the disease.

Dr Craig Spencer, 33, is in isolation at Bellevue Hospital. Two friends and his fiancée have been quarantined. At a news conference, New York Mayor Bill de Blasio sought to reassure the public that they are safe from Ebola.

"Being on the same subway car or living near someone with Ebola does not in itself put someone at risk," De Blasio said.

The first confirmed case in America's largest city set off renewed fears about the spread of the virus, which has killed nearly 4,900 people, largely in Liberia, Sierra Leone and Guinea.

There have been nine cases of Ebola seen in the United States since the beginning of August.

The US stock market reacted to the news and S&P futures fell 9 points or 0.45%. The dollar slipped against the euro and the US 10-year Treasury rose, lowering its yield to about 2.24%.

Spencer developed a fever and gastrointestinal symptoms after working for the humanitarian organisation in Guinea, one of three West African nations hardest hit by Ebola.

He felt the first symptom about 10 am ET on Thursday when he developed a fever, city Health Commissioner Mary Travis Bassett said. People with Ebola only become contagious when they start feeling sick, she said.

He had been monitoring his temperature twice a day, she said.

A specially trained team wearing protective gear transported Spencer to Bellevue Hospital from his Manhattan apartment, the city said in a statement.

He came into close contact with two friends and his fiancee who have been placed in quarantine, she said. One of them is now in the hospital, she said.

A fourth person, a taxi driver, did not come into close contact and was not considered at risk, she said.

Spencer's apartment in Manhattan's Harlem neighbourhood is sealed off, the health commissioner said.

His test will be sent to the Centers for Disease Control for confirmation which should come within 24 hours, she said.

The commissioner said Spencer completed work in Guinea on October 12 and left two days later. Spencer's Facebook page, which included a photo of him clad in protective gear, said he stopped over in Brussels.

Spencer arrived at John F Kennedy International Airport in New York on October 17. After arriving home, he took a 3-mile (4.8-km) run, rode several subways, went bowling and may have eaten at a restaurant, the health commissioner said.

"He was not symptomatic. He had no fever" when he was out, she said.

The first person diagnosed with Ebola on US soil flew from Liberia to Texas and later died in a Dallas hospital. Two nurses who treated him became infected with the virus 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.

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

The United States this week began requiring travellers coming from Liberia, Sierra Leone and Guinea to enter through one of five airports conducting increased screening for the virus. It also is directing those travellers to check in with health officials every day and report their temperatures and any Ebola symptoms for 21 days.

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