The region is close to Rwanda, South Sudan and Uganda, and tens of thousands of people cross those borders every day. Some 20 million have gone back and forth from the outbreak zone since August, Dr. Redfield estimated, and added, “Truly, it’s nearly miraculous that we haven’t seen cross-border spread yet.”

The C.D.C. has worked with the neighboring countries to set up screening stations to stop the disease from reaching them. Some travelers with suspicious symptoms have been tested, but so far none have been infected.

Dr. Redfield said that experts from his agency could do more to help stop the disease, but that so far, because of violence in the area, the United States government had not permitted them to work where they are needed most, in the epicenters of the outbreak. Some were deployed in August to Beni, but were quickly relocated because of unrest in the area. C.D.C. employees are working in other parts of Congo, however, to train health workers and help coordinate the response.

The State Department decides whether it is safe for government employees to work in other countries.

“We’re ready to deploy as soon as they tell us it’s time,” Dr. Redfield said.

He noted that health workers from the World Health Organization, Doctors Without Borders, Alima and other aid groups, had been working nonstop in the region for more than seven months. Fatigue was setting in, he said, and workers needed reinforcements, especially leaders with deep experience in this kind of outbreak.

Several red flags indicate that the outbreak is not under control, Dr. Redfield said. One is that too many people — about 40 percent — are dying at home and never going to treatment centers. There is a high risk that they have infected family members, health workers and other patients at local clinics they might have gone to for help. The disease is spread by bodily fluids and becomes highly contagious when symptoms start.

Corpses are very infectious and pose a big risk to relatives who may wash, dress and prepare them for burial.