The doctor, Craig Spencer, 33, returned from Guinea 10 days ago after treating Ebola patients there. Since returning, Spencer rode the A, L, and No. 1 subway lines, went running, used an Uber livery cab, and went to a bowling alley in Williamsburg called the Gutter Wednesday night, said Dr. Mary Bassett, the city's health commissioner. He also visited the High Line park in Manhattan at some point, she said. He began exhibiting a fever and diarrhea on Thursday morning,

In a statement, Doctors Without Borders said that the doctor and his fiance on Thursday morning followed guidelines by reporting his fever immediately to the Nobel Peace Prize-winning organization, which then alerted the city's Health Department.

"Today, EMS HAZ TAC Units transported a patient to Bellevue Hospital who presented a fever and gastrointestinal symptoms," Basset said in a statement earlier Thursday before the diagnosis. "As a further precaution, beginning today, the Health Department's team of disease detectives immediately began to actively trace all of the patient's contacts to identify anyone who may be at potential risk. The Health Department staff has established protocols to identify, notify, and, if necessary, quarantine any contacts of Ebola cases."

Mayor Bill de Blasio said that the man had limited contact with others since coming back from Africa and was working with health officials to identify them. One of those was his fiance, who was also brought to Bellevue. The mayor also cautioned New Yorkers that their chances of contracting Ebola are very slim, even if you were on the subway with Spencer.

"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."

Spencer is the fourth Ebola patient treated in the United States and the first case in New York City.

The CDC has sent an Ebola response team to New York. The CDC will work to confirm the diagnosis.