PARIS, June 26 -- He crossed two continents and landed at Charles de Gaulle with what felt like a headache. By the time French health authorities confirmed he was carrying the Bundibugyo strain of Ebola virus, he had spent hours on a commercial flight from Kinshasa with more than a hundred other passengers, most of whom had no reason to believe they had been near one of the deadliest pathogens known to medicine. He was a doctor. He had been on a humanitarian mission in the Democratic Republic of Congo. He came home feeling unwell.

France confirmed the case on June 24, the first Ebola infection to reach European soil in the current outbreak, which has already recorded 1,118 confirmed cases and 291 deaths in the DRC as of June 25, making i...