A man who recently visited West Africa was placed in isolation at a city hospital and was undergoing tests for possible Ebola, officials said Monday.

The man, suffering from a high fever and gastrointestinal symptoms, arrived at Mount Sinai Medical Center in Manhattan early on Monday, the hospital said. He had recently traveled to a West African country where Ebola has been reported, it said.

“The patient has been placed in strict isolation and is undergoing medical screenings to determine the cause of his symptoms,” the hospital said in a statement.

Officials at U.S. airports are watching travelers from Africa for flu-like symptoms that could be tied to the recent Ebola outbreak there.

The Ebola virus causes a hemorrhagic fever that has sickened more than 1,600 people, killing nearly 900 mostly in Liberia, Guinea and Sierra Leone. It’s spread through direct contact with bodily fluids, such as blood or urine, unlike an airborne virus such as influenza or the common cold. A person exposed to the virus can take up to 21 days to exhibit any symptoms, making it possible for infected travelers to enter the U.S. without knowing they have it.

Over the weekend, an American physician infected with Ebola was brought to the United States from Africa. He was being treated in Atlanta. A second aid worker was expected to arrive in several days.

Health officials say the threat to Americans at home remains relatively small.

The Centers for Disease Control and Prevention has said three Americans in the United States were tested for Ebola since the West African outbreak erupted this year, and all three results were negative.

Border patrol agents at John F. Kennedy International Airport in New York and Dulles Airport in Washington have been told to ask travelers about possible exposure to the virus and to be on the lookout for anyone with a fever, a headache, achiness, a sore throat, diarrhea, vomiting, stomach pain, a rash or red eyes.

While the CDC says it is not screening passengers boarding planes at African airports—the job of local authorities there—it said it has encouraged vulnerable countries to follow certain precautions. Outbound passengers in the countries experiencing Ebola are being screened for fevers and with health questionnaires.