The study results mean worries about doomsday scenarios, such as the ebola virus becoming transmissible by air, seem unlikely, experts say.

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Fears that the current Ebola epidemic, the deadliest in history, was caused by a more lethal, fast-moving or easily transmissible virus than in previous outbreaks appear to be unfounded, according to a new study.

The study, a genetic analysis published in the journal Science on Thursday, is based on data that indicate the virus has mutated over time in a way that is similar to that of previous, smaller outbreaks.

Researchers say studies of more cases and more recent ones are needed to confirm these findings. But the new analysis offers encouraging evidence that tests used to diagnose Ebola patients, and vaccines and drugs being developed to prevent and treat the disease, can continue to be based on the typical mutation rate. It also means worries about doomsday scenarios, such as the virus becoming transmissible by air, seem unlikely, experts say.

“It hasn’t become increasingly lethal or increasingly virulent,” said David Safronetz, an author of the study and a staff scientist for the Laboratory of Virology at the National Institute of Allergy and Infectious Diseases. “The virus; it’s doing what it’s always done.”

Essentially, the study indicates, while this outbreak has infected 24,000 people and killed about 10,000, its scale has to do with where the epidemic erupted — at the intersection of three vulnerable countries — rather than with any unusual characteristics of the virus.

The scientists evaluated change in the virus over time by comparing genetic-sequencing data from a small number of cases in Mali in October and November with data from patients infected in Guinea last March and Sierra Leone in June.

They found that the number of mutations was about the same as in viruses in previous outbreaks, suggesting the virus was not mutating faster. And they reported that the genetic changes they identified were not significant enough to make the virus more transmissible or deadlier.

Meanwhile, doctors say a U.S. health-care worker who contracted Ebola while volunteering in Sierra Leone has improved from critical to serious condition at the National Institutes of Health (NIH) hospital near Washington.

The patient was flown from Sierra Leone on a chartered plane and arrived March 13. His name and age have not been released. He was working with Partners in Health, a Boston-based nonprofit organization, which has been treating patients in Liberia and Sierra Leone since November.

The organization said 10 other clinicians who had been in contact with the infected worker are being monitored and haven’t shown symptoms.