A U.S. physician who just returned from Guinea has tested positive for the Ebola virus, according to news sources, who say that Dr. Craig Spencer, 33, had been in the West African country working for Doctors Without Borders.

At a news conference, Mayor Bill de Blasio reassured New Yorkers that the city was prepared to deal with Ebola and that “strong protocols” were being followed.

Spencer reportedly arrived Friday at New York's John F. Kennedy International Airport and began feeling tired on Tuesday. While he was asymptomatic, he drove a vehicle in a ride-sharing service, rode on several subway trains and went bowling at a bowling alley in Williamsburg.

City health commissioner, Mary Travis Bassett said subway passengers were in no danger. The bowling alley shut down after Spencer’s diagnosis.

Spencer realized that he had a fever Thursday on Thursday morning, she said. He was subsequently taken to New York City’s Bellevue Hospital.

A Centers for Disease Control and Prevention (CDC) rapid response team arrived in New York last night.

New York Governor Andrew Cuomo said officials had been “realistic” about Ebola appearing in the state. "This is New York. People come to New York," said Cuomo. People come to New York's airports."