A worker from Partners In Health, the prominent American medical aid organization, and an emergency worker from the British military have been infected with the deadly Ebola virus in Sierra Leone, health officials said Thursday.

The Partners In Health worker was the first in that group to be infected since it made an ambitious commitment last fall to help combat Ebola in West Africa, and was the first American health worker in months to get the disease while working in the region. The infections of both the American and Briton served as a reminder that the scourge that has ravaged Sierra Leone, Liberia and Guinea for the past year is far from defeated, even as the number of new cases has declined drastically.

The National Institutes of Health in Bethesda, Md., said in a statement that the American had been ordered flown back to the United States in isolation on a chartered plane, to be admitted on Friday to the N.I.H. hospital in Bethesda. The statement did not identify the worker by name or affiliation, and the person’s precise condition was not known. But other officials in Sierra Leone confirmed the person had been part of a Partners In Health team caring for Ebola patients in the Port Loko district.