In all eight of the previous known Ebola outbreaks in Congo, the virus has been contained within remote jungle villages or relatively small towns, where isolated populations are less likely to spread the disease. But the Congo River is effectively the region’s highway system. Barges and boats travel from Kisangani in the east through major cities including Bumba, Mbandaka — and eventually Kinshasa, the capital of Congo and home to more than 11 million people, as well as Brazzaville, the capital of the Republic of Congo. “The Congo River connects three national capitals and multiple other large cities,” said Jeremy...