MONDAY, March 16, 2015 (HealthDay News) -- The condition of an unidentified American health care worker infected with Ebola in West Africa has been downgraded from serious to critical, doctors at the U.S. National Institutes of Health said Monday.

No other details about the patient, who works with the Boston-based non-profit Partners in Health, were being made available, officials said. The patient is being treated at the NIH Clinical Center in Bethesda, Md.

The worker was flown in isolation on a chartered flight to the United States from Sierra Leone on Friday and was then admitted to the high-level containment facility in Maryland.

The U.S. Centers for Disease Control and Prevention said that, besides the patient in Maryland, 11 other Partners in Health workers have been brought to the United States for monitoring. They include four taken to Atlanta to be near Emory University Hospital; four taken to Nebraska Medical Center in Omaha, and three others who arrived in the Washington, D.C., area on Sunday to be near the NIH hospital in Bethesda, the Associated Press reported.

The health providers were also working in Sierra Leone, one of three West African nations hit hard by the Ebola outbreak that began last year.

The CDC said it continues to conduct "contact tracing" of individuals in Sierra Leone who may interacted with the patient now undergoing treatment in Maryland.

The Maryland patient is the 11th person with Ebola to be treated in the United States since August. Of the first 10 people treated for Ebola in the United States, eight survived and two died.

The death toll in the largest-ever Ebola outbreak has passed 10,000, the World Health Organization reported last week.

Nearly all the victims have been in the West African nations of Guinea, Liberia and Sierra Leone. Only 15 Ebola deaths have occurred in other countries: Mali, Nigeria and the United States, the Associated Press reported.