A doctor in New York City who had recently returned from Guinea has tested positive for the Ebola virus, according to health officials.

Craig Spencer, 33, had been working with Doctors Without Borders to treat Ebola patients in the West African country. He returned to the United States on Oct. 17.

On Thursday, after detecting a fever and gastrointestinal symptoms, he was taken to the city’s Bellevue Hospital where he was tested for the disease.

A follow-up test is now being conducted by the Centers for Disease Control.

Spencer, who works as an emergency room doctor, is now undergoing treatment at the city’s Bellevue Hospital.

“Bellevue Hospital is designated for the isolation, identification and treatment of potential Ebola patients,” according to a statement from the New York City Department of Health.

“New York City is taking all necessary precautions to ensure the health and safety of all New Yorkers.”

New York City Mayor Bill de Blasio said the city was well prepared to handle the treatment of Ebola patients.

“We’ve been preparing for months for the threat posed by Ebola,” he said.

Spencer has told officials he travelled on the subway and went bowling before he showed symptoms for Ebola. An emergency team is now trying to find people who may have come into contact with the doctor.

New York Gov. Andrew Cuomo urged calm, saying “the more facts you know, the less frightening the situation is.”

He said officials have already identified about four people who came into contact with Spencer and has communicated with all of them.

“We’ve already acted very, very quickly,” he said.

Cuomo also stressed that Ebola is not an airborne disease and can only be contracted through the bodily fluids of a patient showing symptoms for the virus.

Spencer’s fiancee is now in isolation as a precaution, and officials have cordoned off his Harlem apartment, according to The Associated Press.