(RNS) At least one of two American medical missionaries diagnosed with the deadly Ebola virus in Liberia could be back in the United States as early as this weekend for treatment at Atlanta's Emory University Hospital.

The hospital has said only that there are plans for an American aid worker to be transferred to its facilities for treatment, but did not name the patient nor provide an arrival time. A medical transport plane left the United States on Thursday (July 31) headed to Liberia, to pick up at least one of the Americans.

CNN reports, however, that two Americans being airlifted from Liberia -- Dr. Kent Brantly and Nancy Writebol. Both are described as being in grave but stable condition.

Brantly, 33, of Fort Worth, was in Liberia for the North Carolina-based missionary group Samaritan's Purse to oversee an Ebola treatment center. Writebol, of Charlotte, N.C., was also working at the center on behalf of Service in Mission.

The Centers for Disease and Control and Prevention issued a travel warning on Thursday (July 31) for all non-essential travelers to Liberia, Guinea and Sierra Leone in West Africa.

Samaritan's Purse said in a statement Thursday that it was working to evacuate all but the most essential personnel to their home country by this weekend, although the center will remain open.

Dr. Margaret Chan, director-general of the World Health Organization, said Friday (Aug. 1) that more than 60 health care workers have lost their lives in helping treat Ebola patients.

WHO says 729 people have died in the latest outbreak of Ebola in the region.

Sierra Leone has declared a statement of emergency, banning public meetings and sending troops door to door to look for new cases and to quarantine the homes of former patients. Liberia has closed its public schools.

In a meeting with the presidents of the three West African countries, Chan the Ebola outbreak "is moving faster than our efforts to control it."

If the situation continues to deteriorate, Chan said, "the consequences can be catastrophic in terms of lost lives, but also several socioeconomic disruption and a high risk of spread to other countries."

There is no vaccine nor specific treatment, which has produced a fatality rate of about 60 percent in the latest cases.

A U.S. citizen, Patrick Sawyer, died last week after arriving in Lagos, Nigeria, on a flight from Liberia. Sawyer, a 40-year old consultant with the Liberian Ministry of Finance, is survived by a wife and three children in Coon Rapids, Minn.

Emory Hospital said it has a specially built isolation unit set up in collaboration with the CDC to treat patients who are exposed to certain serious infectious diseases.

The facility, they said, is physically separate from other patient areas of the hospital and is equipped to provide an extremely high level of clinical isolation. Emory's facility is one of only four of its type in the nation.

CDC Director Dr. Thomas Frieden said he doubted Ebola could spread in the United States. "That is not in the cards," he told reporters.

(Doug Stanglin writes for USA Today.)

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