An American health care worker who arrived in the U.S. early Friday morning after contracting Ebola virus in Sierra Leone is in "serious" condition, the National Institutes of Health, at whose facility the patient is being treated, said in a statement.

Health officials began assessing the patient, whose gender has not been released, Friday morning after the patient was flown back to the United States in isolation on a chartered plane. The patient is being treated at the NIH's Special Clinical Studies Unit in Bethesda, Maryland, a facility designed to handle highly infectious cases. "The unit staff is trained in strict infection control practices optimized to prevent spread of potentially transmissible agents such as Ebola," the NIH said in a statement.

The patient remains unidentified but was reported to be working with Partners In Health, a medical aid organization, and had contracted the virus while working in an Ebola treatment center. This is the tenth case of Ebola in the United States, of which eight have recovered from the deadly virus.

The NIH did not disclose what kind of treatment options it was considering for this newest patient. Patients who have survived Ebola have received a range of treatments, from experimental drugs to transfusions of blood from another individual who has survived the virus.

In 2014, a nurse, Nina Pham, contracted the virus from a patient, Thomas Duncan, at the hospital she worked at in Dallas. Duncan eventually passed away, but Pham, who was treated at the NIH, eventually recovered fully. 

Another foreign health worker, a nurse with the British Army, also contracted Ebola this week in Sierra Leone. She was flown to the United Kingdom early Thursday for treatment. Four other colleagues with whom she came into contact have returned to the U.K. as well and are being watched.