NEW YORK — A Doctors Without Borders physician who recently returned to New York from West Africa has tested positive for the Ebola virus, becoming the first diagnosed case in the city, authorities said late Thursday.

The doctor, identified as Craig Spencer, 33, came back from treating Ebola patients in Guinea October 17 and developed a fever, nausea, pain and fatigue Thursday. He is in isolation and being treated at New York’s Bellevue Hospital, one of the eight hospitals statewide that Gov. Andrew Cuomo designated earlier this month as part of an Ebola preparedness plan.

Spencer, who is hospitalized in intensive care, went for a jog, may have gone to a restaurant, traveled the city’s vast subway system and went bowling before feeling ill, but authorities stressed that the likelihood of him spreading the virus was low.

“We want to state at the outset there is no reason for New Yorkers to be alarmed,” Mayor Bill de Blasio told reporters late Thursday.

Health officials said three people who had been in contact with Spencer — his fiancée and two friends — were healthy and would be quarantined and monitored. A fourth, a car service driver, had no physical contact with the patient and was not considered at risk.

Dr. Mary Travis Bassett, New York City’s health commissioner, said Spencer completed his work in Guinea on October 12 and left Africa two days later via Europe. He arrived at John F. Kennedy Airport on October 17. She said he exhibited no symptoms during his journey or any time afterward until Thursday morning. He had been checking his temperature twice a day.

Spencer went for a three-mile jog and visited a bowling alley in Brooklyn named The Gutter prior to feeling symptomatic Thursday morning, Bassett said. The bowling alley has been closed. He also traveled on three subway lines. Authorities are checking his MetroCard to determine where else he went.

“At the time that the doctor was on the subway he did not have fever … he was not symptomatic,” according to Bassett, who said the chances of anyone contracting the virus from contact with Spencer were “close to nil.”

De Blasio and Bassett were joined by Gov. Cuomo at a news conference to allay concerns about the spread of the virus, especially via public transportation.

“We are as ready as one could be for this circumstance,” Cuomo said, adding that the situation in his state is different than what happened in Texas, where a man from Liberia was diagnosed with Ebola and two health care workers who treated him later contracted the virus.

“We had the advantage of learning from the Dallas experience,” Cuomo said.

De Blasio added, “Ebola is very difficult to contract. Being on the same subway car or living near someone with Ebola does not put anyone at risk.”

The physician, employed at New York’s Columbia Presbyterian Hospital, has been in isolation at Bellevue since he was taken there by emergency personnel Thursday morning.

His Manhattan apartment has been isolated.

Earlier Thursday, de Blasio — without naming the doctor being treated — said that “careful protocols were followed every step of the way” in the city’s handling of the case. The hospitalized doctor has “worked closely” with health officials, the mayor said.

The doctor exhibited symptoms of the Ebola virus for “a very brief period of time” and had direct contact with “very few people” in New York, de Blasio told reporters.

On his Facebook page, Spencer posted a photo of himself in protective gear. The page indicates he went to Guinea around September 18 and later to Brussels in mid October.

“Off to Guinea with Doctors Without Borders (MSF)” he wrote. “Please support organizations that are sending support or personnel to West Africa, and help combat one of the worst public health and humanitarian disasters in recent history.”

In a statement, Columbia Presbyterian Hospital said the doctor was “a dedicated humanitarian” who went to “an area of medical crisis to help a desperately underserved population.”

“He is a committed and responsible physician who always puts his patients first,” the hospital statement said. “He has not been to work at our hospital and has not seen any patients at our hospital since his return from overseas.”

The federal Centers for Disease Control and Prevention had people packing up to go to New York on Thursday, and a specimen from the physician was to be sent to CDC headquarters in Atlanta for testing, an official familiar with the situation told CNN’s Elizabeth Cohen.

In a statement Thursday, Doctors Without Borders confirmed that the physician recently returned from West Africa and was “engaged in regular health monitoring.” The doctor contacted Doctors Without Borders Thursday to report a fever, the statement said.

The doctor began feeling sluggish a couple of days ago, but it wasn’t until Thursday, when he developed 100.3-degree fever, that he contacted Doctors Without Borders, authorities said.

The case came to light after the New York Fire Department received a call shortly before noon Thursday about a sick person in Manhattan. The patient was taken to Bellevue.

Mark Levine, a city councilman who represents the doctor’s Manhattan neighborhood, said earlier Thursday, before news broke of the doctor’s positive test, that city health department workers were canvassing the area, distributing information on the disease door-to-door, according to CNN affiliate WABC.

“The goal right now is to make sure people don’t panic,” he said.

The health department said a special ambulance unit transported a patient suffering from a fever and gastrointestinal symptoms.

Bellevue Hospital is designated for the “isolation, identification and treatment of potential Ebola patients” in the city, the statement said.

“As a further precaution, beginning today (Thursday), the Health Department’s team of disease detectives immediately began to actively trace all of the patient’s contacts to identify anyone who may be at potential risk,” the health department statement said.

“The chances of the average New Yorker contracting Ebola are extremely slim,” the statement said, adding that the disease is spread by direct contact with the bodily fluids of an infected person.

Ebola has killed nearly 5,000 people, mostly in Liberia, Sierra Leone and Guinea. But fears about its spread has mounted since the first person diagnosed with the disease in the United States was hospitalized in Texas last month.

Thomas Eric Duncan, who had flown from Liberia to Dallas, died on October 8. Two nurses who treated him became infected with the virus and are undergoing treatment, with the cases raising questions about the ability of local and federal officials to deal with an outbreak in the United States.

Starting Monday, all travelers coming to the United States from Ebola-affected areas will be actively monitored for 21 days.

In addition, all U.S.-bound passengers from Liberia, Sierra Leone and Guinea must land in one of the five U.S. airports with enhanced screening for Ebola: New York’s John F. Kennedy International, Washington Dulles, New Jersey’s Newark Liberty International, Chicago’s O’Hare International and Hartsfield-Jackson International in Atlanta.