New York City doctor Craig Spencer tested positive for the Ebola virus on Thursday.  This is the fourth confirmed case of Ebola in the United States and the first case in New York City. Government officials emphasized that city has been preparing for a possible outbreak for months.

“There is no reason for New Yorkers to be alarmed,” Mayor Bill de Blasio said at a press conference Thursday night. “It is an extremely hard disease to contact.”

The 33-year-old doctor returned to the city Oct. 17  from a Doctors Without Borders trip to Guinea, where he worked with Ebola patients. He began showing symptoms, including a fever of 103 degrees and gastrointestinal issues, Thursday morning and was immediately put in isolation at Bellevue Hospital, the city’s designated hospital for any cases of the virus.

The Centers for Disease Control and Prevention (CDC) dispatched an Ebola response team to New York City and is expected to conduct an additional test to confirm Thursday’s results, according to the Associated Press.

Although Ebola is not contagious unless the person is symptomatic, the New York City Department of Health is investigating the possibility that Spencer could have spread the virus and infected others around the city. Spencer came in contact with four people, Governor Andrew Cuomo said Thursday. Spencer's fiancée has also been placed in a quarantine ward at the hospital, but she has not shown any Ebola symptoms. 

“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,” according to a statement from the NYC Health Department released Thursday.

Bellevue Hospital has a team of “disease detectives” who have been “putting together pieces of the timeline,” de Blasio said. Spencer traveled from Manhattan to Brooklyn using the subway Wednesday night, according to the New York Times. He went to a bowling alley in Williamsburg and then took the car service Uber back to his apartment in upper Manhattan.

Unlike other U.S. cities who have been cases of Ebola, New York has been preparing for months and is “as ready as one could be for this circumstance,” Cuomo said. “What happened in Dallas was actually the complete opposite.”