(Bloomberg) — A New York City doctor has tested positive for Ebola after returning from aid work in West Africa, spurring authorities to track his movements and city and state officials to reassure the public the risk of contracting the disease is minimal.

It is the first case of Ebola diagnosed in the most populous U.S. city. The doctor, Craig Spencer, 33, is being treated in an isolation unit at Bellevue Hospital Center in Manhattan. At the same time, officials are monitoring those who were with him during a time when he traveled on the subway, went bowling and had close contact with three friends.

Spencer arrived in New York Oct. 17, and came down with a temperature yesterday. His case puts a spotlight on how tough it may be to control Ebola’s spread, potentially spurring new debate on how to protect the U.S. It also supports recent efforts to prepare for such cases.

“We are as ready as one can be for this circumstance,” said New York Governor Andrew Cuomo, one of several officials who spoke out yesterday to lessen city fears. “New York is a dense place, a lot of people on top of each other. But the more you know, the less frightening it is.”

Spencer, an emergency medicine doctor at Columbia University-New York Presbyterian Hospital, is the second person to be diagnosed with Ebola in the U.S. after arriving from West Africa, where the disease has infected more than 10,000 people, killing about half. Thomas Eric Duncan died from the disease in a Dallas hospital after infecting two nurses who remain hospitalized.

Guinea travel

Spencer returned to New York from Guinea, one of three countries where the Ebola outbreak has been raging, after working there with Doctors Without Borders. He hasn’t been attending to patients at the hospital since returning, New York Presbyterian said in a statement.

Spencer began feeling tired on Oct. 21, and first noticed he had a fever sometime about 10 a.m. yesterday. He then immediately alerted Doctors Without Borders, New York City Health Commissioner Mary Bassett said. The aid group contacted health authorities and Spencer was brought to Bellevue, a state- designated center for such cases, in a special ambulance.

“The individual engaged in regular health monitoring and reported this development immediately,” said Tim Shenk, a Doctors Without Borders spokesman, by e-mail.

Cuomo said health authorities believe only four people had close contact with Spencer during the “relevant period” while he may have exposed others to the infection.

Bowling trip

On Oct. 22, Spencer went to The Gutter, a bowling alley in the Williamsburg section of Brooklyn, City Health Commissioner Mary Bassett said. Spencer had also traveled on the A, 1 and L subway lines while he’s been in New York, officials said.

The finance and friends are healthy but under quarantine, according to Bassett. The Uber driver had no direct physical contact, she said.

Uber said it contacted health officials and were told “that neither our driver partner, nor any of his subsequent passengers are at risk,” spokeswoman Kristin Carvell said in statement. “We have communicated this to the driver, and the NYC DOHMH medical team met with the driver in person, assuring him that he is not at risk.”

The bowling alley closed “out of an abundance of caution,” Bassett said. In Spencer’s Harlem neighborhood, workers were seen handing out informational fliers about the illness.

Symptoms onset

Bassett said she considered the fever the onset of symptoms that would make the patient contagious. The fatigue, she said, could have been attributed to any number of reasons.

“He limited his contact with people,” Bassett said, although before he was showing symptoms he left his apartment and, among other things, rode the the subway trains

The Ebola diagnosis was made by the New York City Department of Health & Mental Hygiene. Samples of Spencer’s blood will be sent to the U.S. Centers for Disease Control and Prevention in Atlanta for confirmation, the CDC said.

“There is no reason for New Yorkers to be alarmed, Ebola is an extremely hard disease to contract,” said New York City Mayor Bill de Blasio at a news conference announcing the infection. “New Yorkers who haven’t been expose to an infected person’s bodily fluids aren’t at all at risk.”

CDC team

A CDC ‘Go’ team was dispatched to New York, even before Spencer was formally diagnosed with the disease. It will help health workers at Bellevue safely care for him while trying to track down anybody he may have had close contact with while he was sick, said Tom Skinner, an agency spokesman.

The CDC team is “composed of epidemiologists, infection control, data management, and communications officers, he said.

New York state officials have been planning for an Ebola patient for months, Cuomo said, and recently ordered hospitals to go through practice drills and enhance safety protocols.

Bellevue, an 828-bed hospital, has been designated as one of four facilities for Ebola treatment in New York, a city of 8.4 million residents. The facility has four single-bed rooms in its infectious-disease ward to receive high-probability or confirmed cases and a laboratory to handle blood samples, according to New York City Health and Hospitals Corp.

Mount Sinai Hospital and New York Presbyterian in Manhattan, and Montefiore Medical Center in the Bronx, are the other New York City facilities for patients. Four others are on Long Island and in Syracuse and Rochester.

Facebook postings

Spencer left Guinea Oct. 14 and arrived at John F. Kennedy International Airport on Oct. 17, Bassett said. He posted a picture on his Facebook page on Sept. 18, saying he was going to Guinea with Doctors Without Borders.

‘‘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,’’ read the caption of a photo showing a man wearing a protective gown, face shield, eye mask and gloves.

The West Africa outbreak has infected about 10,000 people, mostly in Liberia, Sierra Leone and Guinea, killing half, according to the World Health Organization. Yesterday, an Ebola case was confirmed in Mali, which shares a border with Guinea.

There’s no approved cure for Ebola in the U.S., though some patients have received experimental drugs. The current standard of care involves supporting patients by replacing fluids and electrolytes, and using antibiotics to fight off any opportunistic infections.

Fever, chills

Early symptoms can be similar to those of malaria such as fever and chills. Ebola also causes headache, stomach pain, weakness, diarrhea, vomiting, and bleeding. During later stages, patients can lose many liters of bodily fluid each day. The virus spreads through direct contact with a victim.

Patients are most infectious at late stages when large quantities of the virus are present inside the body. This explains why health-care workers and relatives tending to the gravely ill are most at risk of infection.

Two nurses in Dallas were infected at Texas Health Presbyterian Hospital while caring for Duncan, the first man to die of the virus in the U.S. One of the nurses, Amber Vinson, 29, is now free of the virus. She is being treated at Emory University Hospital in Atlanta. The other nurse, Nina Pham, 26, is in good condition at a National Institutes of Health medical center in Maryland.

The CDC said on Oct. 22 that anyone traveling to the U.S. from Liberia, Sierra Leone or Guinea would be monitored for 21 days, the maximum incubation period for the virus. Travelers leaving the countries are already checked at the airport before boarding a plane, and about 36,000 people have been screened.

Despite strict safety measures by Doctors Without Borders, 14 of their staff members have been infected with Ebola and eight have died since March, the organization said Sept. 24. The greatest risk is believed to be when they are out in the community rather than when working with patients, the group said.

–With assistance from Caroline Chen, Doni Bloomfield, Brian Chappatta, Martin Z. Braun, Allyson Versprille and Cynthia Koons in New York and John Lauerman in Atlanta.