Dr. Frieden said that a major concern is that travelers who become ill or injured and need medical care may risk being exposed to Ebola at hospitals in the region. When infection-control measures are poor, hospitals become “amplification points,” spreading the disease they are supposed to contain, he said.

The White House press secretary, Josh Earnest, said Thursday that the United States was considering a medical evacuation to bring home American aid workers diagnosed with Ebola. Two, infected in Liberia, were in grave condition, according to a statement on the website of a charitable organization, Samaritan’s Purse.

Emory University Hospital in Atlanta issued a statement on Thursday saying it had been told of plans to transfer a patient with Ebola to its special containment unit, set up in collaboration with the C.D.C., in the next several days. Because of privacy laws, the hospital declined to name the patient.

The W.H.O. said the tally of deaths from Ebola rose after 56 more people died during the four days ending Sunday in Guinea, Liberia and Sierra Leone. Nigeria had one fatality. In the same period, 122 new cases were detected, bringing the total of confirmed and suspected cases to 1,323.

The virus causes high fever, vomiting, diarrhea and, in some cases, bleeding. In the current outbreak, about 60 percent of the cases have been fatal. It is transmitted by contact with bodily fluids from someone who is ill.