Scientists have found smallpox viruses in the remains of people in Chile from around 500 years ago, revealing how the virus may have evolved into a global killer.

The patient had traveled to the Democratic Republic of Congo, the health ministry said. Workers are racing to trace those who may have had contact.

That is good news for patients, but officials fear it will make controlling the spread of the disease harder.

The Bundibugyo virus, a little known type, previously had caused just two small outbreaks. Now it’s at the center of a rapidly widening epidemic in Africa.

The types of Ebola and hantavirus worrying officials are very different from the species identified decades ago, raising new questions about how to respond.