Story highlights Official: Ebola patient left Africa on October 14, arrived in U.S. on the 17th

He checked temperature twice daily, wasn't symptomatic until Thursday, official adds

It is New York City's first diagnosed case of Ebola

The 33-year-old physician was working with Doctors Without Borders

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.

JUST WATCHED Mayor: Working to protect all New Yorkers Replay More Videos ... MUST WATCH Mayor: Working to protect all New Yorkers 02:25

JUST WATCHED Gov. Cuomo: We're as ready as one can be Replay More Videos ... MUST WATCH Gov. Cuomo: We're as ready as one can be 04:28

JUST WATCHED Doctor tested for Ebola is Craig Spencer Replay More Videos ... MUST WATCH Doctor tested for Ebola is Craig Spencer 01:46

Spencer posted this image to Facebook on September 18 from Brussells, saying "Off to Guinea with Doctors Without Borders (MSF). 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."

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.

Photos: The Ebola epidemic Photos: The Ebola epidemic An Ebola survivor participates in a study in Monrovia, Liberia, on June 17, 2015. The country launched a five-year study to unravel the mystery of the long-term health effects that plague survivors of the viral disease. Since the epidemic started more than a year ago in a remote village in Guinea, more than 11,000 people have died, the vast majority in three West African nations, according to the latest numbers from the World Health Organization . And that number is believed to be low, since there was widespread under-reporting of cases, according to WHO. Hide Caption 1 of 45 Photos: The Ebola epidemic Women in Monrovia celebrate after the World Health Organization declared Liberia Ebola-free on May 9, 2015. Other cases have recurred since, however. Two people in Liberia have died of the disease since the end of June, just weeks after the WHO declared the nation free of the disease. Hide Caption 2 of 45 Photos: The Ebola epidemic A man walks past an Ebola awareness painting in Monrovia on March 22, 2015. Hide Caption 3 of 45 Photos: The Ebola epidemic Soldiers from the U.S. Army's 101st Airborne Division walk across the tarmac at Campbell Army Airfield before reuniting with their families at a homecoming ceremony March 21, 2015 in Fort Campbell, Kentucky. The 162 soldiers were deployed in Liberia, where they helped fight the spread of Ebola. Hide Caption 4 of 45 Photos: The Ebola epidemic Relatives weep for a loved one who it was believed died from Ebola, at a graveyard on the outskirts of Monrovia on March 11, 2015. Hide Caption 5 of 45 Photos: The Ebola epidemic Doctors Without Borders staffer Alex Eilert Paulsen watches as mattresses and bed frames burn at the Ebola Treatment Unit in Paynesville, Liberia, on January 31, 2015. The organization reduced its number of beds from 250 to 30 as gains were made in battling the virus. Hide Caption 6 of 45 Photos: The Ebola epidemic Pauline Cafferkey, a Scottish woman diagnosed with Ebola, is put on a plane in Glasgow, Scotland, on December 30, 2014. Cafferkey, a 39-year-old nurse who volunteered in Sierra Leone, was being transported to London for treatment. Hide Caption 7 of 45 Photos: The Ebola epidemic A child who survived the Ebola virus is fed by another survivor at a treatment center on the outskirts of Freetown, Sierra Leone, on November 11, 2014. Hide Caption 8 of 45 Photos: The Ebola epidemic Health workers in Monrovia cover the body of a man suspected of dying from the Ebola virus on October 31, 2014. Hide Caption 9 of 45 Photos: The Ebola epidemic Kaci Hickox leaves her home in Fort Kent, Maine, to take a bike ride with her boyfriend on October 30, 2014. Hickox, a nurse, recently returned to the United States from West Africa, where she treated Ebola victims. State authorities wanted her to avoid public places for 21 days -- the virus' incubation period. But Hickox, who twice tested negative for Ebola, said she would defy efforts to keep her quarantined at home. Hide Caption 10 of 45 Photos: The Ebola epidemic Health officials in Nairobi, Kenya, prepare to screen passengers arriving at the Jomo Kenyatta International Airport on October 28, 2014. Hide Caption 11 of 45 Photos: The Ebola epidemic U.S. President Barack Obama hugs Ebola survivor Nina Pham in the Oval Office of the White House on October 24, 2014. Pham, one of two Dallas nurses diagnosed with the virus, was declared Ebola-free after being treated at a hospital in Bethesda, Maryland. The other nurse, Amber Vinson (not pictured), was treated in Atlanta and also declared Ebola-free. Hide Caption 12 of 45 Photos: The Ebola epidemic Health workers in Port Loko, Sierra Leone, transport the body of a person who is suspected to have died of Ebola on October 21, 2014. Hide Caption 13 of 45 Photos: The Ebola epidemic Health workers bury a body on the outskirts of Monrovia on October 20, 2014. Hide Caption 14 of 45 Photos: The Ebola epidemic Garteh Korkoryah, center, is comforted during a memorial service for her son, Thomas Eric Duncan, on October 18, 2014, in Salisbury, North Carolina. Duncan, a 42-year-old Liberian citizen, died October 8 in a Dallas hospital. He was in the country to visit his son and his son's mother, and he was the first person in the United States to be diagnosed with Ebola. Hide Caption 15 of 45 Photos: The Ebola epidemic Boys run from blowing dust as a U.S. military aircraft leaves the construction site of an Ebola treatment center in Tubmanburg, Liberia, on October 15, 2014. Hide Caption 16 of 45 Photos: The Ebola epidemic Aid workers from the Liberian Medical Renaissance League stage an Ebola awareness event October 15, 2014, in Monrovia. The group performs street dramas throughout Monrovia to educate the public on Ebola symptoms and how to handle people who are infected with the virus. Hide Caption 17 of 45 Photos: The Ebola epidemic Ebola survivors prepare to leave a Doctors Without Borders treatment center after recovering from the virus in Paynesville, Liberia, on October 12, 2014. Hide Caption 18 of 45 Photos: The Ebola epidemic A man dressed in protective clothing treats the front porch of a Dallas apartment on October 12, 2014. The apartment is home to one of the two nurses who were diagnosed with Ebola after treating Thomas Eric Duncan, a Liberian national who traveled to Dallas and later died from the virus. Hide Caption 19 of 45 Photos: The Ebola epidemic A woman crawls toward the body of her sister as a burial team takes her away for cremation October 10, 2014, in Monrovia. The sister had died from Ebola earlier in the morning while trying to walk to a treatment center, according to her relatives. Hide Caption 20 of 45 Photos: The Ebola epidemic A man digs a grave on October 7, 2014, outside an Ebola treatment center near Gbarnga, Liberia. Hide Caption 21 of 45 Photos: The Ebola epidemic A person peeks out from the Dallas apartment where Thomas Eric Duncan, the first person diagnosed with the Ebola virus in the United States, was staying on October 3, 2014. Hide Caption 22 of 45 Photos: The Ebola epidemic A girl cries as community activists approach her outside her Monrovia home on October 2, 2014, a day after her mother was taken to an Ebola ward. Hide Caption 23 of 45 Photos: The Ebola epidemic A health official uses a thermometer September 29, 2014, to screen a Ukrainian crew member on the deck of a cargo ship at the Apapa port in Lagos, Nigeria. Hide Caption 24 of 45 Photos: The Ebola epidemic Workers move a building into place as part of a new Ebola treatment center in Monrovia on September 28, 2014. Hide Caption 25 of 45 Photos: The Ebola epidemic Medics load an Ebola patient onto a plane at Sierra Leone's Freetown-Lungi International Airport on September 22, 2014. Hide Caption 26 of 45 Photos: The Ebola epidemic A few people are seen in Freetown during a three-day nationwide lockdown on September 21, 2014. In an attempt to curb the spread of the Ebola virus, people in Sierra Leone were told to stay in their homes. Hide Caption 27 of 45 Photos: The Ebola epidemic Supplies wait to be loaded onto an aircraft at New York's John F. Kennedy International Airport on September 20, 2014. It was the largest single shipment of aid to the Ebola zone to date, and it was coordinated by the Clinton Global Initiative and other U.S. aid organizations. Hide Caption 28 of 45 Photos: The Ebola epidemic A child stops on a Monrovia street September 12, 2014, to look at a man who is suspected of suffering from Ebola. Hide Caption 29 of 45 Photos: The Ebola epidemic After an Ebola case was confirmed in Senegal, people load cars with household items as they prepare to cross into Guinea from the border town of Diaobe, Senegal, on September 3, 2014. Hide Caption 30 of 45 Photos: The Ebola epidemic A health worker wearing a protective suit conducts an Ebola prevention drill at the port in Monrovia on August 29, 2014. Hide Caption 31 of 45 Photos: The Ebola epidemic A burial team from the Liberian Ministry of Health unloads bodies of Ebola victims onto a funeral pyre at a crematorium in Marshall, Liberia, on August 22, 2014. Hide Caption 32 of 45 Photos: The Ebola epidemic Dr. Kent Brantly leaves Emory University Hospital on August 21, 2014, after being declared no longer infectious from the Ebola virus. Brantly was one of two American missionaries brought to Emory for treatment of the deadly virus. Hide Caption 33 of 45 Photos: The Ebola epidemic An Ebola Task Force soldier beats a local resident while enforcing a quarantine on the West Point slum on August 20, 2014. Hide Caption 34 of 45 Photos: The Ebola epidemic Local residents gather around a very sick Saah Exco, 10, in a back alley of the West Point slum on August 19, 2014. The boy was one of the patients that was pulled out of a holding center for suspected Ebola patients after the facility was overrun and closed by a mob on August 16. A local clinic then refused to treat Saah, according to residents, because of the danger of infection. Although he was never tested for Ebola, Saah's mother and brother died in the holding center. Hide Caption 35 of 45 Photos: The Ebola epidemic Workers prepare the new Ebola treatment center on August 17, 2014. Hide Caption 36 of 45 Photos: The Ebola epidemic Liberian police depart after firing shots in the air while trying to protect an Ebola burial team in the West Point slum of Monrovia on August 16, 2014. A crowd of several hundred local residents reportedly drove away the burial team and their police escort. The mob then forced open an Ebola isolation ward and took patients out, saying the Ebola epidemic is a hoax. Hide Caption 37 of 45 Photos: The Ebola epidemic A health worker disinfects a corpse after a man died in a classroom being used as an Ebola isolation ward August 15, 2014, in Monrovia. Hide Caption 38 of 45 Photos: The Ebola epidemic Aid worker Nancy Writebol, wearing a protective suit, gets wheeled on a gurney into Emory University Hospital in Atlanta on August 5, 2014. A medical plane flew Writebol from Liberia to the United States after she and her colleague Dr. Kent Brantly were infected with the Ebola virus in the West African country. Hide Caption 39 of 45 Photos: The Ebola epidemic Members of Doctors Without Borders adjust tents in the isolation area in Kailahun on July 20, 2014. Hide Caption 40 of 45 Photos: The Ebola epidemic Boots dry in the Ebola treatment center in Kailahun on July 20, 2014. Hide Caption 41 of 45 Photos: The Ebola epidemic Dr. Jose Rovira of the World Health Organization takes a swab from a suspected Ebola victim in Pendembu, Sierra Leone, on July 18, 2014. Hide Caption 42 of 45 Photos: The Ebola epidemic Red Cross volunteers disinfect each other with chlorine after removing the body of an Ebola victim from a house in Pendembu on July 18, 2014. Hide Caption 43 of 45 Photos: The Ebola epidemic A scientist separates blood cells from plasma cells to isolate any Ebola RNA and test for the virus April 3, 2014, at the European Mobile Laboratory in Gueckedou, Guinea. Hide Caption 44 of 45 Photos: The Ebola epidemic Health specialists work March 31, 2014, at an isolation ward for patients at the facility in southern Guinea. Hide Caption 45 of 45

JUST WATCHED NYC doctor being tested for Ebola Replay More Videos ... MUST WATCH NYC doctor being tested for Ebola 02:20

JUST WATCHED Family: Infected nurse is Ebola-free Replay More Videos ... MUST WATCH Family: Infected nurse is Ebola-free 01:43

An image of Craig Spencer taken from his LinkedIn profile.

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.