An American health care worker who contracted Ebola while volunteering in a Sierra Leone treatment unit has been downgraded to critical condition at the National Institutes of Health, doctors said Monday.

The agency said in a statement that the patient's status was changed from serious condition. He is being treated at the National Institutes of Health's hospital near Washington.

"We are intensively treating the patient," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at NIH. "He's in our special clinical studies unit and, hopefully, that will be able to turn this around and the patient will recover, but it's too early to say."

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The patient was flown in isolation from Sierra Leone on a chartered plane last week and arrived early Friday morning. His name and age have not been released.

The man is a clinician working with Partners in Health, a Boston-based nonprofit organization. The group has been treating patients in Liberia and Sierra Leone since November.

The latest NIH patient is the 11th person with Ebola to be treated in the U.S. Two patients in the U.S. have died: a man treated in Dallas after contracting the virus in Africa and a doctor evacuated from Africa to Nebraska when he was already critically ill.

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The man in Dallas had contracted the virus in his native Liberia. He transmitted the disease to two nurses, resulting in widespread concern in the U.S., with questions raised about emergency department screening of patients, monitoring of ill travelers from Africa and even disputes over the disposal of potentially infectious waste from hospitals.

The World Health Organization has estimated the virus has killed more than 10,000 people, mostly in the West African nations of Guinea, Liberia and Sierra Leone. The current outbreak is the largest ever for the disease. While deaths have slowed dramatically in recent months, the virus appears stubbornly entrenched in parts of Guinea and Sierra Leone.

Besides the man at NIH, there are 12 other Partners in Health workers being brought to the United States for monitoring. A Centers for Disease Control and Prevention spokesman said that includes four going to Atlanta to be near Emory University Hospital; one arrived Friday. On Saturday, four health care workers arrived at the Nebraska Medical Center in Omaha; another was expected Monday. Three arrived in the Washington area on Sunday to be near the NIH campus in Bethesda.

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One of those being monitored in Nebraska has developed symptoms of Ebola and was being moved to an isolation unit, hospital officials said in a news release Monday. Nebraska Medical Center spokesman Taylor Wilson said the individual developed symptoms Sunday evening and was hospitalized as a precaution. Wilson declined to describe the symptoms, but said they had resolved Monday.

None of those being monitored has tested positive for Ebola.

The National Institutes of Health said it has no other pending admissions of additional patients with the Ebola virus or who have been exposed to Ebola.

CDC workers in Sierra Leone are involved in investigating the illness of the first patient, including looking for other people the person was in contact with. It's possible other people will be transported to the United States for monitoring, said the spokesman, Tom Skinner.