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.
Normally, Ebola circulates among animal hosts, probably African bats. Scientists suspect that the West African epidemic began when a bat infected a boy in a village in Guinea in December 2013.
As reports of the outbreak surfaced, Dr. Pardis C. Sabeti, a computational biologist at Harvard, and her colleagues started a collaboration with doctors in Sierra Leone. The researchers quickly sequenced the genomes of 99 Ebola viruses isolated from 78 patients there.
Their analysis showed that Ebola was moving quickly from one victim to the next, and that the virus was gaining new mutations along the way. One worrying possibility was that those mutations somehow sped up Ebola's replication.
The mutation, the scientists found, made the viruses much more successful at attacking human cells and those of other primates. Compared with the older gene, the mutated form infected four times as many primate cells.
But the mutation did not help the hybrid viruses infect the cells of other species, such as cats and dogs.
It is not clear what role the mutation played in West Africa's epidemic.
But the fact that Ebola did gain at least one advantage that made it better at infecting human cells worries scientists anyway. We will almost certainly face another outbreak in the future.