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A species of Ebola virus that emerged in Uganda in November 2007 is unlike any other, scientists report in the November PLoS Pathogens. A team of U.S. and Ugandan researchers collected and analyzed blood samples from people infected in and around the town of Bundibugyo and found that last year’s outbreak of hemorrhagic fever there resulted from a previously unknown Ebola species, tentatively called Bundibugyo ebolavirus. The virus infected roughly 100 people, of whom 37 died, says virologist Jonathan Towner of the Centers for Disease Control and Prevention in Atlanta.

The scientists show that the Ugandan Ebola species harbors genetic material that differs from that of its more virulent cousins, Ebola-Zaire and Ebola-Sudan. Those two species of Ebola have caused sporadic but deadly outbreaks across Central Africa in recent decades. While the genetic difference certainly explains why the Uganda outbreak had a lower fatality rate than these other species, “We still don’t know exactly why they are so virulent,” Towner says. Meanwhile, the new finding also raises the question of whether vaccines being developed for Ebola-Zaire and Ebola-Sudan will work against the Ugandan species. CDC researchers have provided vaccine developers with their genetic findings. —Nathan Seppa