Mayor Bill de Blasio said that a doctor admitted to Bellevue Hospital Center on Thursday has tested positive for the Ebola virus, adding “there’s no reason for New Yorkers to be alarmed.”

A doctor in New York City who recently returned from treating Ebola patients in Guinea became the first person in the city to test positive for the virus Thursday, setting off a search for anyone who might have come into contact with the man while he was sick.

The doctor, Craig Spencer, was rushed to Bellevue Hospital Center and placed in isolation at the same time as investigators sought to retrace every step he took for the past several days.

At least three people he had contact with in recent days have been placed in isolation. The federal Centers for Disease Control and Prevention, which dispatched a team to New York, is conducting its own test to confirm the positive test on Thursday, which was performed by a city lab.

While officials have said they expected isolated cases of the disease to arrive in New York eventually, and had been preparing for this moment for months, the first case highlighted the challenges surrounding containment of the virus, especially in a crowded metropolis. Dr. Spencer had traveled on the A and L subway lines Wednesday night, visited a bowling alley in Williamsburg, then took a taxi back to Manhattan. .