Members of the media react as NYPD officers enter the building where Dr. Craig Spencer lives in New York, Oct. 24, 2014. Spencer, a doctor who worked in West Africa with Ebola patients, was in an isolation unit in New York City on Friday after testing positive for the virus, becoming the fourth person diagnosed with the disease in the United States and the first in its largest city. Reuters

Before he was hospitalized, Ebola patient Dr. Craig Spencer returned from West Africa to New York and spent a week visiting some of the city's trendiest neighborhoods and landmarks, potentially coming into contact with millions of New Yorkers. He rode the subway, went bowling at a hipster hideaway in Brooklyn and took a subway into Manhattan. His busy social life upon returning home to Harlem after a month in Guinea could complicate an ongoing investigation to find and isolate everyone who came into contact with him when he might have been infectious in a city of 8 million people.

Spencer, 33, contracted the virus while working with the group Doctors Without Borders. He traveled to Africa through Brussels in mid-September and returned on Oct. 16 to his apartment on West 147th Street in Harlem, according to the New York Times. He was placed in isolation at Bellevue Hospital Thursday after reporting a fever and gastrointestinal symptoms. A hazardous materials team sanitized his apartment Friday and his fiancée, Morgan Dixon, who lived with him, has been quarantined at Bellevue. Two other friends whom he had contact with have also been quarantined. Spencer also came into contact with an Uber driver, according to New York City Health Commissioner Dr. Mary Bassett.

Ebola is spread by direct contact with the bodily fluids of an infected person. The outbreak has killed nearly 5,000 people, mostly in Liberia, Sierra Leone and Guinea. U.S. health officials have said it's virtually impossible to contract Ebola. Spencer is the fourth person diagnosed with Ebola in the United States. A Liberian national who traveled to Dallas spread the deadly virus there to two nurses before he died. "There is no reason for New Yorkers to be alarmed," New York City Mayor Bill de Blasio said Thursday. "New Yorkers who have not been exposed to an infected person's bodily fluids are not at all at risk."

Before he became ill, Spencer celebrated his return to New York by crisscrossing the city and stopping at some of its more popular sites. Health officials said Spencer took the A, L and 1 subway trains Wednesday and hung out at The Gutter bowling alley in Williamsburg. He also ate at a restaurant and visited the High Line park in Manhattan. Spencer works at Columbia Presbyterian Hospital in Manhattan, but he hadn't seen any patients since his return, according to CNN.

"He is a committed and responsible physician who always puts his patients first," the hospital said in a statement. "He has not been to work at our hospital and has not seen any patients at our hospital since his return from overseas. Our thoughts are with him, and we wish him all the best at this time."

City health department workers swarmed Spencer's neighborhood Friday, passing out fliers with information about Ebola and its symptoms. "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," Sam Miller, associate commissioner of the New York City Department of Health and Mental Hygiene, told CNN.