President Obama pledged federal support to New York on Thursday night after a doctor there tested positive for Ebola days after returning to the United States from the epicenter of the epidemic in West Africa.

Obama promised New York Mayor Bill de Blasio (D) and Gov. Andrew Cuomo (D) additional support in caring for Craig Spencer. The 33-year-old patient remained in isolation late Thursday at Bellevue Hospital, officials said. He was taken to the hospital earlier in the day after reporting a fever and gastrointestinal symptoms.

Spencer, who had been treating Ebola patients in Guinea, is the fourth person diagnosed with Ebola in the United States — and the first diagnosed outside Texas.

“We were hoping that it didn’t happen, but we were also realistic,” Cuomo said in a news conference Thursday night. “This is New York. People come through New York. People come through New York’s airports, so we can’t say this is an unexpected circumstance. We are as ready as one could be for this circumstance.”

City health commissioner Mary Bassett said that Spencer had completed his work in Guinea on Oct. 12 and departed the country on Oct. 14. He arrived at New York’s John F. Kennedy International Airport on Oct. 17, after a stop in Brussels.

Bassett said that Spencer had remained well throughout that journey but began feeling sluggish on Tuesday. Still, she said, he went on a three-mile jog and rode on at least three New York subway lines — the A, L and 1 trains. She said he also went to a bowling alley in Williamsburg on Wednesday and rode home using the Uber car service. The bowling alley has closed “out of an abundance of caution,” she said.

Spencer registered a fever of 100.3 degrees between 10 a.m. and 11 a.m. Thursday, New York health officials said. Initial reports that his temperature was 103 degrees were incorrect, Cuomo told Fox 5 News on Friday.

Already on Thursday, a team of disease detectives covered the city, tracking down anyone who may have had contact with Spencer. The Centers for Disease Control and Prevention sent a team of specialists in epidemiology, infection control and communications to New York on Thursday evening.

Bassett said officials had identified three at-risk contacts — Spencer’s fiancee and two other friends — all of whom are well but have been placed under quarantine. They also have been in touch with the Uber driver, though he is not considered a high-risk contact.

Health officials were quick to remind city residents that the chances of the average New Yorker contracting Ebola are extremely slim. The deadly virus is spread only through contact with the bodily fluids of an Ebola patient, typically after symptoms appear.

“There is no reason for New Yorkers to be alarmed,” de Blasio told reporters at Thursday night’s news conference. “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.”

He and Cuomo assured residents that workers at hospitals throughout the city have been conducting safety drills and reviewing proper protocols in recent weeks in the event Ebola showed up on their doorsteps.

This is the largest outbreak of the Ebola virus in history. (The Washington Post/The Washington Post)

“Dallas was unfortunately caught before they could prepare,” said Cuomo, referring to the missteps that happened when Thomas Eric Duncan, a Liberian man who arrived at a Dallas hospital with Ebola last month, was initially misdiagnosed. “We had the advantage of learning from Dallas’s experience.”

Cuomo also underscored that Spencer himself is a doctor who had seen Ebola up close, had been monitoring himself and immediately recognized the symptoms. “He was familiar with the possibility and the circumstances, so he handled himself accordingly,” Cuomo said.

Before Spencer, three people had been diagnosed with Ebola in the United States. Duncan died on Oct. 8 after a week in the hospital. Two nurses who treated him, Nina Pham and Amber Vinson, also were infected. Pham is being treated at a National Institutes of Health facility in Bethesda, Md. Vinson is at Emory University Hospital in Atlanta; her family said this week that she appears to be free of the virus.

Earlier this month, Cuomo designated Bellevue as one of eight hospitals in the state that could care for potential Ebola patients. An official said Thursday evening that any hospital in the city should be prepared to identify and isolate potential Ebola patients.

Spencer works as a doctor at New York-Presbyterian/Columbia University Medical Center. On Thursday, the hospital called him “a dedicated humanitarian” who “always puts his patients first.” The hospital said in a statement that he had not been to work and had not seen patients since returning from overseas.

On Sept. 18, Spencer wrote on his Facebook page that he was heading to Guinea — one of three West African nations in the throes of an unprecedented Ebola outbreak — to work with Doctors Without Borders, the international relief group that has been on the front lines of the crisis.

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.

The New York-Presbyterian/Columbia University Web site says Spencer has worked in the past on “multiple projects in East Africa,” including helping develop an emergency care teaching curriculum in Rwanda. He also spent time working in Burundi and in Congo.

Late Thursday, a media swarm descended on the brick-and-stone six-story apartment building where Spencer lives in a busy section of Harlem. Residents of the building said that the building supervisor called each of the building’s 200 to 300 tenants to inform them that Spencer was being hospitalized.

“I feel sorry for the individual: I feel sorry for his family,” said Alvin McCain, who was walking his aunt home to her apartment on West 147th Street. But he added: “This is going to take New York into a frenzy.”

Word of a potential Ebola case in New York came as Mali’s health minister confirmed that country’s first case Thursday. The patient is a 2-year-old girl who recently traveled from Guinea, according to government officials.

Since the beginning of the outbreak, the World Health Organization has counted nearly 10,000 likely cases of Ebola in eight countries: Guinea, Liberia, Sierra Leone, Nigeria, Senegal, Mali, Spain and the United States. The vast majority are in Guinea, Sierra Leone and Liberia.

The official death toll for the outbreak has risen to nearly 5,000, though the actual loss of life is probably far higher.

Wesley Lowery in New York and Abby Ohlheiser, Lena H. Sun, DeNeen Brown, Juliet Eilperin and Lindsey Bever in Washington contributed to this report.