“This is where everything gets more complicated,” said Chiran Livera, the operation leader in Congo for the International Federation of Red Cross and Red Crescent Societies.

The villages surrounding Bikoro and Iboko are among the most isolated and densely wooded pockets of Congo. Aid workers must use motorbikes to navigate cratered dirt roads that flood during the rainy season. Maps of some regions are incomplete, and vast gaps in cellular service thwart efforts to report data to central operations.

“Following the virus’s narrative may sounds easy to do on a suburban street outside Chicago,” said Dr. Salama. “But when you’re traveling hundreds of kilometers in a forest by motorbike to find each person, that’s very different epidemiological work.”

If the outbreak worsens, a second vaccination may be offered to health workers. That vaccine, developed by Johnson and Johnson, requires two doses and would take longer than VSV-EBOV’s seven to 10 days to become effective — but may protect health workers for several years.

The Congolese Ministry of Health is planning to deploy up to five experimental treatments, though the two most highly recommended by the W.H.O. may prove impractical in a remote setting.

ZMapp, a cocktail of three antibodies used in West Africa, must be given in multiple doses and must be refrigerated. Remdesivir, a drug developed by Gilead Sciences, requires intensive monitoring of liver and kidney function — nearly impossible for treatment centers without electricity, running water or standard equipment.