Officials say an American health care worker who contracted Ebola while volunteering in Sierra Leone has improved to fair condition at a federal hospital.

The National Institutes of Health announced Monday that the patient had been upgraded from serious condition. He was in critical condition earlier.

He is being treated at the NIH hospital in Bethesda, Maryland, where he arrived March 13.

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

Health officials are monitoring about 40 other Americans who may have been in contact with the infected man. None tested positive as of late last week. The 21-day monitoring period for all of them ends this week.