The Ebola epidemic that tore through West Africa in 2014 claimed 11,310 lives, far more than any previous outbreak. A combination of factors contributed to its savagery, among them a mobile population, crumbling public health systems, official neglect and hazardous burial practices.

But new research suggests another impetus: The virus may have evolved a new weapon against its human hosts. In studies published on Thursday in the journal Cell, two teams of scientists report that a genetic mutation may have made Ebola more deadly by improving the virus’s ability to enter human cells.

The researchers do not yet understand exactly how it works, but several lines of evidence suggest it helped expand the scope of the epidemic. One alarming finding: Patients infected with the mutated version of Ebola were significantly more likely to die.

“It’s hard to escape the conclusion that it’s an adaptation to the human host,” said Dr. Jeremy Luban, a virologist at the University of Massachusetts Medical School and an author of one of the new studies.