A U.S. healthcare worker who tested positive for the Ebola virus while in Sierra Leone  was in serious condition at a Maryland medical facility and a  second American who may have been exposed to that patient was being flown back to the United States, U.S. health officials said on Friday.

The National Institutes of Health said the U.S. Ebola patient was flown into the United States earlier on Friday and admitted to the NIH's high-security containment facility in Maryland. The patient is the 11th person with the deadly virus treated in the United States.

The NIH said the patient was in serious condition. The NIH did not release any more details.

The aid group Partners In Health said in a statement that the clinician was working for them in Sierra Leone, and noted their colleague "remains in good spirits."

The U.S. Centers for Disease Control and Prevention said another American volunteering in Sierra Leone had "potential exposure" to that patient and was being transported to the Atlanta area to be near Emory University Hospital, which has treated other Ebola patients.

The developments followed a relatively quiet period for Ebola in the United States, a reminder that while the spread of the virus has eased somewhat in West Africa, it still remains dangerous.

The CDC said that as a result of the latest case it is working to trace the contacts of volunteers combating Ebola in Sierra Leone, including several other Americans, who may have been exposed to the healthcare worker now at the NIH.

The CDC said none of these other people, including the one headed to Atlanta, has tested positive.

However, the CDC said it was working with the State Department to develop plans to return those Americans with potential exposure to the United State, where they will isolate themselves and be under direct CDC monitoring 21 days.

A British healthcare worker who tested positive for Ebola while in Sierra Leone was flown back this week to Britain, along with four others who are being monitored for possible infection.

CDC spokesman Benjamin Haynes said the agency's team in Sierra Leone is still gathering information but said there is no evidence so far that the U.S. and British cases are related.

While the virus has killed about 10,000 people in Sierra Leone, Liberia and Guinea, only a handful of cases have been seen in the United States, Spain and Britain.