Health officials say four more American aid workers have arrived back in the U.S. from West Africa to be monitored for Ebola.
Tuesday's arrivals bring to 15 the number of aid workers who have returned from Sierra Leone. They will be isolated and monitored during the next three weeks for signs of the disease.
All came in contact with an American colleague who came down with Ebola last week while working in Africa for Partners in Health, a Boston-based international aid organization. That man is in critical condition at a National Institutes of Health hospital in Bethesda, Md.