"[A]s scientists we need to be very careful we don’t scaremonger"

It is conceivable that Ebola might become more deadly during this outbreak, but it is by no means a certainty. Ebola outbreaks typically last only months, but other viruses have needed decades to make the change.

Like its close relatives, Ebola spreads through infected fluids, such as vomit and blood. There is no firm evidence that the strain that has caused human outbreaks can spread through the air.

Over the course of millions of years, viruses do sometimes switch their route of infection. “It does happen in an evolutionary context,” Dr. Holmes said.

But it would be a mistake, he warned, to imagine that with a single mutation Ebola might become an airborne pathogen. The change would require many mutations in many genes, and it might be nearly impossible for so many mutations to emerge during a single outbreak. The mutated viruses would survive only if they were superior to the ones spread by bodily fluids.