The entire W.H.O. unit devoted to the science of pandemic and epidemic diseases — responsible for more than a dozen killers, including flu, cholera, yellow fever and bubonic plague — has only 52 regular employees, including secretaries, according to its director, Dr. Sylvie Briand, who said that could be increased during outbreaks. Before the Ebola epidemic, her department had just one technical expert on Ebola and other hemorrhagic diseases.

Across Africa, the ranks of the agency’s regional emergency outbreak experts, veterans in fighting Ebola, were cut from more than a dozen to three. “How can you immediately respond to an outbreak?” said Dr. Francis C. Kasolo, a W.H.O. director. “It did affect us.”

And a separate section of the W.H.O. responsible for emergency response was whittled “to the bone” during the budget cuts — to 34 staff members from about 94 — according to Dr. Bruce Aylward, its assistant director general.

“You can’t make a cut that big, that deep, and it’s not going to have an effect on your operational capacity,” he said.

His group, charged with responding to wars, disasters and resurgent polio, was asked in August to assist with Ebola, too. “At no time that I can think of in the recent past have we been dealing with such a scale of human misery over such a broad geography due to such a range of hazards,” he said, including enormous population displacements in Syria, Iraq, the Central African Republic and South Sudan. But, officials warn, multiple, overlapping challenges may well be a feature of the future.

The W.H.O. hoped to balance its budget cuts by strengthening the ability of countries to respond to public health threats on their own. It put out new regulations for nations to follow to help contain outbreaks. But by 2012, the deadline it set, only 20 percent of nations had enacted them all. In Africa, fewer than a third of countries had programs to detect and stop infectious diseases at their borders. The W.H.O.'s strategy was often more theory than reality.

“There never were the resources to put those things in place in many parts of the world,” said Dr. Scott F. Dowell, a specialist formerly with the C.D.C.