NAIROBI, July 2 -- The child was a year and a half old, living in Kyegegwa district in western Uganda, and by the time laboratory results confirmed what had killed her, she was already gone. Health officials in Kampala and Geneva now say the pathogen was Marburg virus, a hemorrhagic fever with no licensed vaccine and no approved treatment, arriving in a country that is already exhausted from weeks of fighting a different, and larger, outbreak of Ebola.

The finding, reported to the World Health Organization on June 30, is not a new epidemic on paper. It is one confirmed case, no known onward transmission, and a government in Kampala that, as of Wednesday, would not confirm it is happening at all. But it lands in the middle of an Ebola cri...