WASHINGTON - The National Institutes of Health is preparing to care for an American doctor who was exposed to the Ebola virus while volunteering in Sierra Leone.

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Out of what the agency called "an abundance of caution,'' the physician is expected to be admitted to the special isolation unit at the NIH's hospital near Washington, D.C., as early as Sunday, for observation.

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,'' said an agency statement.

Four other Americans aid workers who were infected with Ebola while volunteering in the West African outbreak have been treated at hospitals in Georgia and Nebraska. One remains hospitalized while the rest have recovered.