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 Dr. 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 W. Geisbert at the University of Texas Medical Branch in Galveston, and Dr. Pardis C. Sabeti at the Broad Institute of Harvard and M.I.T. 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 their victims run out of hosts and fade out.

But Dr. Sabeti said she has no gene data showing that that is happening in West Africa, especially since scientists are not yet 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 another 95 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.