A New York City doctor has tested positive for Ebola after returning from a humanitarian trip to Africa last week.

Thirty-three year old Craig Spencer worked with Doctors Without Borders in Guinea to treat those affected by the deadly virus. When he returned to the U.S. on October 17, he showed no signs of the virus and took several precautionary measures, including taking his temperature twice a day.

But early Thursday morning, Spencer developed a 100.3-degree fever, nausea, pain, and fatigue. He's currently being treated in an isolation ward at Manhattan's Bellevue Hospital, which is a designated Ebola center set up with the necessary equipment to treat the virus.

Despite the fact that Spencer spent several days in public after his return home from Africa doing everything from riding the subway to bowling, New York health officials want to make sure that the citizens of New York City understand that they're not at risk unless they were directly exposed to the doctor.

"We want to state at the outset that New Yorkers have no reason to be alarmed," said Mayor Bill de Blasio in a press conference. "New Yorkers who have not been exposed are not at all at risk."

Since Ebola isn't contagious until the symptoms begin, health officials are placing three people into quarantine today: Spencer's fiancee and two friends. The doctor saw no patients at his Columbia Presbyterian Hostpital office after his return home.

New York City health department workers are passing out fliers at Spencer's apartment complex letting neighbors know that they're not at risk. 

"What we're doing now is just telling the folks who live here in the neighborhood that they're safe. It's safe for them to be in their buildings, it's safe for them to go to their apartments, it's safe for them to walk down the street," said Sam Miller, associate commissioner of the New York City Department of Health and Mental Hygiene.

Craig Spencer is the fourth person to be diagnosed with Ebola in the United States, and the tenth American to test positive for the virus worldwide.