A new discovery about the Ebola virus is quite terrifying: scientists have found that the virus was able to adapt and mutate. A simple mutation of the Ebola gene made the virus more infectious during the epidemic in West Africa that killed scores of people and led to a worldwide panic.

The studies, published in the journal Cell, determined that this single mutation started early int he epidemic and allowed Ebola to spread more easily than previous versions of the virus. This helps explain why Ebola spread so quickly and so far in West Africa when it showed in 2013, considering the fact that it’s not a new disease. It helped that it struck densely populated cities and West Africa lacks the tools to stop its spread, but this mutation made it all the more deadly.

It’s not uncommon for a virus to mutate, particularly when it first starts to spread. It represents a big concern for scientists, as one method to stop a virus can suddenly become obsolete in the midst of an outbreak.

“There was this belief that Ebola virus essentially never changes,” said TSRI infectious disease researcher Kristian G. Andersen, who also serves as director of infectious disease genomics at the Scripps Translational Science Institute (STSI). “But this study tells us that a natural mutation in Ebola virus–which occurred during an outbreak–changed infectivity of human cells.”