NEW YORK  ― An emergency room doctor who recently returned to the city after treating Ebola patients in West Africa has tested positive for the virus, becoming the first case in the city and the fourth in the nation.

Mayor Bill de Blasio and Gov. Andrew Cuomo on Thursday urged residents not to be alarmed by the doctor’s Ebola diagnosis, even as they described him riding the subway, taking a cab and bowling. De Blasio said all city officials followed “clear and strong” protocols in their handling and treatment of him.

“We want to state at the outset that New Yorkers have no reason to be alarmed,” de Blasio said. “New Yorkers who have not been exposed are not at all at risk.”

The doctor, Craig Spencer, a member of Doctors Without Borders who had been working in Guinea, returned six days ago and reported Thursday morning coming down with a high fever and diarrhea. He was being treated in an isolation ward at Manhattan’s Bellevue Hospital, a designated Ebola center.

The Centers for Disease Control and Prevention, which will do a further test to confirm the initial results, has dispatched an Ebola response team to New York, and the city’s disease detectives have been tracing Spencer’s contacts to identify anyone who may be at risk. The city’s health commissioner, Mary Bassett, said Spencer’s fiance and two friends had been quarantined but showed no symptoms.

In the days before Spencer fell ill, he went on a long jog, went to the High Line park, rode the subway and, on Wednesday night, got a taxi to a Brooklyn bowling alley. Bassett said he felt fatigued Wednesday but not feverish until Thursday morning.

Health officials say the chances of the average New Yorker contracting Ebola, which is spread through direct contact with the bodily fluids of an infected person, are slim. Someone can’t be infected just by being near someone who is sick with Ebola. Someone isn’t contagious unless he is sick.

Bassett said the probability was “close to nil” that Spencer’s subway ride would pose a risk. Still, the bowling alley was closed as a precaution, and Spencer’s Harlem apartment was cordoned off. The Department of Health was on site across the street from the apartment building Thursday night, giving out information to area residents.

President Barack Obama had been briefed on the situation in New York, the White House said.

The Ebola epidemic in West Africa has killed about 4,800 people. In the United States, the first person diagnosed with the disease was a Liberian man, who fell ill days after arriving in Dallas and later died, becoming the only fatality. None of his relatives who had close contact with him got sick. Two nurses who treated him were infected and are hospitalized.

According to a rough timeline provided by city officials, Spencer felt fatigue Wednesday and when he felt worse Thursday he and his fiance made a joint call to authorities to detail his symptoms and his travels. EMTs in full Ebola gear arrived and took him to Bellevue in an ambulance surrounded by police squad cars.

Doctors Without Borders, an international humanitarian organization, said per the guidelines it provides its staff members on their return from Ebola assignments, “the individual engaged in regular health monitoring and reported this development immediately.” (AP)