Her colleagues at Doctors Without Borders — often called M.S.F. for its French name, Médecins Sans Frontières — are overwhelmed, she said. In one center, until recently, five people were caring for 100 patients. During crises, many doctors work for the group on rotations of up to six weeks, for which they get small stipends.

The supply of volunteers is drying up. Although the group operates in war zones — including Gaza, Iraq, Syria, and Ukraine — West Africa is the hardest to staff, Dr. Liu said.

“The reality is, it’s difficult to recruit for Ebola,” she said. “You have to learn to live with fear.” And the treatment of Ebola is exhausting. When Dr. Liu worked in Syria, she said, there were lulls after aircraft dropped barrel bombs and the wounded were treated in the emergency room. Ebola patients need constant care.The W.H.O. said Tuesday that there were “encouraging signs“ in Nigeria and Guinea even though 113 new cases were reported in those countries and the two other affected nations. That brought the total number of cases to 2,240, with more than 1,200 deaths.