Health and New York City officials were trying Friday morning to follow the trail of a young emergency room doctor who traveled about the city for three days before being hospitalized for the Ebola virus.

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 100.3-degree fever and diarrhea. He was being treated in an isolation ward at Manhattan's Bellevue Hospital, a designated Ebola center.

In the days before Spencer fell ill, he went on a 3-mile 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.

The bowling alley was shuttered Thursday night, and issued a statement saying, "We've been in constant contact with the Health Department and they have determined that there was no risk to our customers."

Spencer rode the subway in the past five days, traveling on the A, L and 1 lines, a law-enforcement source told The New York Post.

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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."

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.

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. President Barack Obama spoke to Cuomo and de Blasio Thursday night and offered the federal government's support. He asked them to stay in close touch with Ron Klain, his "Ebola czar," and public health officials in Washington.

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 fiancee and two friends had been quarantined but showed no symptoms.

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.

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 fiancee 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."

"Contagion is everywhere," said an official in Guinea for Doctors Without Borders, also known by its French name Medecins Sans Frontieres or MSF. "Even in Conakry, the risk isn't zero. But MSF takes measures to secure its personnel." 

The official refused to say where or how long Spencer had been working in Guinea. The official spoke on condition of anonymity because of the sensitivity of the situation. 

Doctors Without Borders, which has been treating patients since the outbreak was identified in March, runs two treatment centers in Guinea: one in Gueckedou, in the country's southeast, where the outbreak began, and the other in the capital, Conakry. In Macenta, near Gueckedou, it also runs a transit center where patients are screend for Ebola and then sent on for treatment if they are confirmed to have the disease. 

Travelers from Guinea, Liberia and Sierra Leone must report in with health officials daily and take their temperature twice a day, as Spencer did.

Spencer works at NewYork-Presbyterian/Columbia University Medical Center. He had not seen any patients or been to the hospital since his return, the hospital said in a statement, calling him a "dedicated humanitarian" who "went to an area of medical crisis to help a desperately underserved population."

Four American aid workers, including three doctors, were infected with Ebola while working in Africa and were transferred to the U.S. for treatment in recent months. All recovered.

Health care workers are vulnerable because of close contact with patients when they are their sickest and most contagious. In West Africa this year, more than 440 health workers have contracted Ebola and about half have died.

But the Ebola virus is not very hardy. The CDC says bleach and other hospital disinfectants kill it. Dried virus on surfaces survives only for several hours.

Spencer, 33, is from Michigan and attended Wayne State University School of Medicine and Columbia's University Mailman School of Public Health.

According to his Facebook page, he left for West Africa via Brussels in mid-September. A photo shows him in full protective gear. He returned to Brussels Oct. 16.

"Off to Guinea with Doctors Without Borders," 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."

The Associated Press contributed to this report