“The reasons are really unclear,” said Dr. Gilles van Cutsem, who helped run the response in Liberia by Doctors Without Borders and gave a presentation describing its experience at an AIDS conference in Seattle.

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As the Ebola epidemic in West Africa wanes, physicians from Doctors Without Borders are confronting a mystery: More patients are surviving and they do not know why.

“The reasons are really unclear,” said Dr. Gilles van Cutsem, who helped run the agency’s response in Liberia and gave a presentation describing its experience at an AIDS conference in Seattle.

Doctors Without Borders — better known by its French name, Médecins Sans Frontières — has cared for more Ebola patients in West Africa than any other organization. At its peak, it was running 22 centers; it now runs eight.

Since last March, the average death rate at those remaining centers has dropped to 52 percent, from about 62 percent. Though patients are getting more intravenous hydration and more nursing care as staff members have more time, the agency doesn’t think that accounts for the whole difference.

Rather, patients are arriving with less virus in their blood. Viral loads have dropped by almost half, van Cutsem said, which increases a patient’s chances of survival enormously. Initially, he said, the assumption was that patients were coming in earlier in the course of their illnesses. Rumors that everyone who entered Ebola centers died were once rife, but now have faded.

But that was not the case, either. According to interviews with patients, the time between the first appearance of symptoms and the day they arrive has not changed.

“That leaves us with two hypotheses,” van Cutsem said. One is that fear has made West Africans more careful, and that even those who are infected have gotten smaller amounts of virus into their eyes or mouths. Perhaps they are wearing gloves when they bury bodies, he said, or at least partially protecting themselves while caring for sick relatives.

In many diseases, the size of a viral dose may make the difference between life and death. Even though viruses multiply, immune systems handle small amounts of virus better than large ones.

The other theory is that the virus has mutated to be less lethal. But van Cutsem said he knew of no genetic evidence to prove that.

Doctors fighting some previous Ebola outbreaks have had the impression that the virus weakened because more people infected late in the outbreak survived, he said. Two Ebola experts, Thomas Geisbert at the University of Texas Medical Branch in Galveston, and Dr. Pardis Sabeti at the Broad Institute of Harvard and the Massachusetts Institute of Technology in Cambridge, Mass., said that, though the idea is plausible, they knew of no genetic evidence of it happening, largely because so few samples have been sequenced. The conventional wisdom is that viruses slowly mutate toward less lethality because the strains that kill all of their victims run out of hosts and fade out.

But Sabeti said she has no gene data showing that that is happening in West Africa, especially since scientists are not even sure which mutations make Ebola more or less lethal — as they do, for example, with flu.

Her team has published the genomes of nearly 200 virus samples collected during the current outbreak, 99 of them in August and 95 more this week. All came from one hospital in Sierra Leone. Other than that, she said, she knew of “only eight or nine” other new sequences posted by others.

Struggles over the ownership of blood samples and even nasal swabs taken in foreign countries have become more common in recent years, especially when they may lead to vaccines.

During the H5N1 bird-flu scare of a decade ago, Indonesia stopped letting samples be sent to the West, complaining that Indonesians could not afford the vaccines made from them. During the early days of the recent MERS outbreak in Saudi Arabia, struggles over credits on academic papers held up sequencing of samples.

Sabeti said in an interview that she had heard that other teams had received samples but were waiting to publish articles about them in academic journals. “With everything that’s going on, it’s unfortunate that we know so little,” she said.