A New York City physician who recently returned from the front lines of the Ebola epidemic in West Africa has tested positive for the deadly virus, Mayor Bill de Blasio announced at a news conference Thursday night.

The man, identified as Craig Spencer by New York City Councilman Mark Levine, is in isolation at Bellevue Hospital in Manhattan. Spencer, who had been treating Ebola patients in Guinea with Doctors Without Borders, returned to New York last week.

He becomes just the fourth person diagnosed with Ebola in the United States -- and the first diagnosed outside of Texas.

With this new case of Ebola come new fears about the spread of the deadly virus in the most populous city in the country.

"Ebola right now can spread fear just by the sound of the word," New York Gov. Andrew M. Cuomo (D) said at the news conference. "I know it's a frightening situation. ... But the more facts you know the less frightening the situation is."

Said de Blasio: "There is no reason for New Yorkers to be alarmed. Ebola is an extremely hard disease to contract. ... Being on the same subway car or living near a person with Ebola does not in itself put someone at risk."

Spencer, whose name was not mentioned during the news conference at Bellevue Hospital, left Guinea on Oct. 14 and arrived in the United States three days later, said Mary Travis Bassett, the city's health commissioner. During his travels and upon arriving in New York, she said, "he was well with no symptoms."

He checked his temperature twice daily, she said, and "began feeling somewhat tired" on Tuesday, but was well enough to go on a three-mile jog this week.

On Thursday morning, she said, Spencer began to develop a fever, at which point he contacted Doctors Without Borders officials. Soon thereafter, he was transported from his residence in Harlem to the hospital in Manhattan.

His positive diagnosis came several hours later.

The Centers for Disease Control and Prevention has dispatched a team of specialists for epidemiology, infection control and communications to New York.

Well before the positive diagnosis, the city announced that a team of disease detectives had already begun to trace Spencer's contacts, in a mad scramble "to identify anyone who may be at potential risk." At the news conference, Bassett said Spencer had used New York's subway system and went to a bowling alley in Williamsburg on Wednesday. (The bowling alley has closed "out of an abundance of caution," she said.)

Spencer also took an Uber taxi after returning from Guinea, but had no contact with the driver. Uber said in a statement that after consulting with public health officials, neither the driver nor his subsequent passengers were deemed to be at risk. The company did not say whether the vehicle had been decontaminated.

Health officials were most concerned with three close contacts -- Spencer's fiance and two friends. "All three of these contacts are healthy and are being quarantined," Bassett said. One of the three, she said, is in the hospital; the other two are under quarantine at home.

None of the three contacts are symptomatic, Bassett said.

"When someone gets Ebola, they are not very infectious initially, but they become increasingly infectious the sicker they get," said CDC Director Tom Frieden.

Frieden added: "Ebola is a scary disease, and it's fearsome because of how severe the illness is, but it does not spread easily."

The deadly virus is spread by direct contact with the bodily fluids of a symptomatic Ebola patient.

The New York lab that conducted the preliminary Ebola test is a part of the Laboratory Response Network, a group of facilities designed to coordinate quickly with the CDC in response to public health threats. The CDC is expected to confirm the results in the next 24 hours.

According to Spencer’s public LinkedIn profile, he has worked as a doctor at NewYork-Presbyterian/Columbia University Medical Center, not far from where he lives, since July 2011.

NewYork-Presbyterian/Columbia University Medical Center didn’t name Spencer, but in a statement described the hospitalized doctor as “a dedicated humanitarian” who hasn’t returned to work nor seen patients since his overseas trip. “He is a committed and responsible physician who always puts his patients first,” the hospital statement read.

On Sept. 18, Spencer published a photo of himself on Facebook wearing personal protective equipment. In an accompanying post, he wrote: "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."

Here's how the virus spreads and how contact tracing works to stop outbreaks. (Gillian Brockell/The Washington Post)

Physicians volunteering with Doctors Without Borders follow strict protocols as they return from the Ebola zone. They first travel through Europe and are debriefed in Brussels. Doctors can remain in the field for a maximum of four to six weeks; upon returning to the United States, they are told to follow CDC guidelines. Those without any known exposure to Ebola are told to monitor their health for a 21-day incubation period, according to the organization.​

Emergency officials received a call just before noon on Thursday for a sick person in Harlem, a fire department spokesman said.

The patient met Ebola risk criteria, so a special hazardous EMS unit was sent to the apartment with personnel who were fully covered in personal protective equipment. The vehicle was immediately decontaminated, said New York City EMS union president Israel Miranda.

The city health department noted that Bellevue Hospital "is designated for the isolation, identification and treatment of potential Ebola patients by the City and State." Earlier this month, Cuomo designated Bellevue as one of eight hospitals in the state that could care for potential Ebola patients.

President Obama is aware of Spencer’s case and is being updated, according to White House official who asked for anonymity because of the sensitivity of the matter.

Three people had been diagnosed with the Ebola virus in the United States before Spencer.

The first was Thomas Eric Duncan, a Liberian man who traveled from West Africa to Dallas through Brussels; Duncan died on Oct. 8 at a hospital in Texas.

Two nurses who treated him, Nina Pham and Amber Vinson, were subsequently diagnosed with the virus. Pham is being treated at a National Institutes of Health facility in Maryland, and Vinson is being treated at Emory University Hospital in Atlanta. In a statement on Wednesday, Vinson's family said that she is now free of the virus.

Five other American patients have been treated for the Ebola virus after being evacuated from West Africa. All five have survived and been discharged from the Emory facility and the Nebraska Medical Center.

The deadliest Ebola outbreak in history has killed nearly 5,000 people in three West African countries, Guinea, Liberia and Sierra Leone. Doctors Without Borders, also known by its French name Médecins Sans Frontières, has been battling the epidemic in rural Guinea, where the outbreak began in March, for months and was one of the earliest aid groups on the ground. Cases and deaths in Guinea have been far lower than in Sierra Leone and Liberia.

According to Spencer’s public LinkedIn profile, he attended Johns Hopkins University and studied Chinese language and literature at Henan University in China. He later went to medical school at Wayne State University in Detroit and earned a master’s degree in public health at Columbia University.

On Thursday night in Harlem, as Spencer's diagnosis was being announced at the news conference at the hospital, Alvin McCain was walking his aunt home to her apartment. She lives directly across West 147th Street from Spencer's six-story apartment building, where a media mob had gathered.

"I feel sorry for the individual, I feel sorry for his family," McClain said. "This is going to take New York into a frenzy!... Who knows what he touched?! Who knows where he has been?"

Said his aunt, Joyce Harrison: "They said it's not airborne, but who knows."

Still, Harrison said, Ebola's arrival in New York won't change much.

"I'll go right on with my daily plans," she said, "and pray to God that it doesn't come my way."

Wesley Lowery, Ed O'Keefe, Abby Ohlheiser, Brady Dennis, DeNeen Brown and Juliet Eilperin contributed to this report. Lowery reported from New York.

[This post has been updated multiple times.]