Doctors say an American health care worker who contracted Ebola while volunteering in a Sierra Leone treatment unit has improved to serious condition at the National Institutes of Health.

The agency announced Thursday that the patient's status improved from critical condition. He is being treated at the NIH's hospital near Washington.

The patient was flown in isolation from Sierra Leone on a chartered plane and arrived March 13. The NIH said initially that the patient was in serious condition, but announced last week that his status had been downgraded to critical.

The patient's name and age have not been released. He is a clinician working with Partners in Health, a Boston-based nonprofit organization, which has been treating patients in Liberia and Sierra Leone since November.