At NYU Langone Medical Center last week, a patient who went to the emergency room with a fever and who mentioned a recent visit to West Africa was given a mask and moved to a secluded area, said Dr. Michael Phillips, the hospital’s director of Infection Prevention and Control. But further questioning revealed that the patient had not visited any of the affected countries, “so we stopped right there,” Dr. Phillips said.

At Bellevue Hospital Center last week, a patient was placed in isolation, but it quickly became clear that he did not have Ebola.

Image Dr. David Reich, left, and Dr. Jeremy Boal at a news conference on Monday at Mount Sinai Hospital. Credit... Robert Stolarik for The New York Times

An Ebola outbreak centered mainly in three West African countries — Sierra Leone, Guinea and Liberia — has infected more than 1,300 people and killed more than 700 of them. American health officials have advised against nonessential travel to the three countries, and have urged doctors to be on high alert for people who return from the region with symptoms like fever, diarrhea and vomiting.

A Mount Sinai spokeswoman, Dorie Klissas, said that to protect the patient’s privacy, the hospital was not making public his occupation, which country he had been in, whether he had been exposed to a patient with Ebola there, or whether he had close contacts like family members, friends or co-workers who were also at risk. Officials said they expected the results of the tests for Ebola in 24 to 48 hours.