(AP) -- New York City's mayor has visited the Manhattan hospital isolation unit where a doctor who contracted Ebola is being treated.

After Sunday's visit, Mayor Bill de Blasio said New Yorkers shouldn't be alarmed about contracting the disease that has killed nearly 5,000 people, mostly in West Africa.

Dr. Craig Spencer is being treated for Ebola at Bellevue Hospital Center after returning from working in Guinea with infected patients. His fiancee has been cleared of the disease but is still being kept under quarantine at the couple's Harlem home.

De Blasio says Spencer was "looking better" on Sunday. Doctors say he's entered the next stage of Ebola, which involves gastrointestinal problems.

The mayor and health officials note that Ebola is difficult to contract. It has to be transmitted through bodily fluids.