An American doctor who was exposed to the Ebola virus while volunteering in Sierra Leone was admitted Sunday to a hospital at the National Institutes of Health near the nation's capital.

The patient, who was not identified, arrived at NIH's Clinical Center about 4 p.m., NIH said in a statement on its website.

NIH said that out of "an abundance of caution," the physician was admitted to a special isolation unit. NIH infectious disease chief Dr. Anthony Fauci wouldn't discuss details about the patient but said that in general, an exposure to Ebola doesn't necessarily mean someone will become sick.

"When someone is exposed, you want to put them into the best possible situation so if something happens you can take care of them," Fauci said.

"NIH is taking every precaution to ensure the safety of our patients, NIH staff and the public," the agency said in a statement.

Four other American aid workers who were sickened by Ebola while volunteering in the West African outbreak have been treated at hospitals in Georgia and Nebraska. One remains hospitalized while the others have recovered.

An Associated Press photographer saw a person dressed in a white protective suit get off a plane and walk to a waiting ambulance at the Frederick Municipal Airport in Maryland about 3:30 p.m. Sunday.

The plane's tail number matched that of the aircraft that has been used previously to transport other Ebola patients to the United States from overseas.

NIH spokesman John Burklow confirmed that the plane carrying the patient landed at Frederick.