By Ed Yong
In December 2013, in a small village in Guinea, the Ebola virus left its traditional host—probably a bat—and infected a young boy. That leap triggered what became the largest Ebola outbreak in history. At first, the virus stayed within Guinea’s borders and, as in every previous epidemic, affected just a few hundred people. But in the spring of 2014, that gentle simmer came to a disastrous boil. Cases skyrocketed as the virus spread to Sierra Leone, Liberia, and other countries. By the time it was finally brought to heel in 2016, more than 28,000 people had been infected and 11,000 of them were dead.
The unprecedented scale of the outbreak gave the virus ample opportunities to adapt to its new human hosts—and it took advantage of them. Two independent teams of scientists have shown that in early 2014, Ebola virus picked up a mutation called A82V, which made it worse at infecting bat cells, but better at infecting human ones. And once viruses with that mutation appeared, they quickly took over. They were the ones that spread beyond Guinea, the ones responsible for the vast majority of cases, the ones that landed in a Texan emergency room.
Both groups emphasize that the mutation may not have caused the virus’s exponential spread. “Its appearance coincided with the virus taking off, but others factors were probably more important, like the movement of infected people into urban areas and lack of proper burials,” says Jonathan Ball from the University of Nottingham, who led one of the teams. “If we hadn’t seen that mutation, there would probably still have been a big outbreak.”
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