Ebola, with no precautions, in the wild, doubles every 21 days, on average. Ebola in the US, with full infectious disease precautions and hazmat gear, doubled in 21 days. Then we put all infectees into Level IV hazmat isolation. That, and the fact that Duncan was the only contagious person to slip out of W. Africa and into the US, is the only reason the disease didn’t go all Black Death on us. Pure, dumb luck.
That city( Butembo) has an Airport. An airport with international flights. Including Boston.