The fourth American aid worker sickened with the Ebola virus arrived Tuesday morning to a mostly calm scene at Emory University Hospital, where two others have been successfully treated.

An ambulance carrying the patient arrived about 10:25 a.m., with a police escort. Wearing a bulky protective suit similar to those of Emory's first two arrivals, the patient walked from the ambulance to the hospital - though a different entrance was used this time around. Reporters and television cameras - but fewer curious onlookers - lined the street.

About an hour earlier, the specially equipped plane carrying the patient touched down at an air base just northwest of Atlanta.

The patient will be housed in a special isolation unit, hospital officials said. The patient's identity was not released. The hospital released no additional details on the patient's identity or status, citing confidentiality restrictions.

But the World Health Organization says a doctor who has been working in an Ebola treatment center in Sierra Leone has tested positive for the disease. It said the doctor was in stable condition Monday in Freetown and was being evacuated.

The Ebola outbreak sweeping West Africa has killed more than 2,000 people and has taken a particularly high toll on health care workers.

Last month, two U.S. aid workers who contracted Ebola in Liberia, Dr. Kent Brantly and Nancy Writebol, were treated successfully at Emory.

Another worker, Dr. Rick Sacra, 51, of Worcester, Massachusetts, is being treated at the Nebraska Medical Center in Omaha. He is in stable condition. Federal officials say they asked that hospital to treat him instead of Emory to prepare other isolation units for more Ebola patients if needed.

Brantly and Writebol were given the experimental drug ZMapp. Both credited the drug with helping their recovery, but there is no way to know its effects. Sacra is being treated with a different experimental drug. His doctors have refused to name it but say they've been consulting with experts on Ebola.

Details of the latest patient's treatment are not known, but ZMapp could not have been used. Brantly and Writebol were the first to receive it; it had never been tested on humans. The rest of the limited supply was given to five others.

Once a new batch is ready, it still needs basic tests before it can be tried again, officials have said.

At Emory on Tuesday, law student Grace Van Dyke said she had heard that some people around the country were initially concerned about Ebola patients being brought to the U.S. But she never heard worries from the university community.

"Those of us who are at Emory, we're not concerned because we know the quality of Emory medical care, and we know the reason they were brought here is because Emory is capable of containing it and treating them," she said.

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(Photo: AP Photo/David Tulis)