An Ebola outbreak now occurring in Guinea was almost certainly started by someone who survived West Africa’s historic 2014-16 epidemic, harbored the virus for at least five years and then transmitted it via semen to a sex partner, researchers reported on Friday.

The finding, based on genetic sequencing of virus samples taken from patients in the current outbreak, shocked researchers. Until now, the longest the virus had been known to persist in a survivor was 500 days.

“It’s a stunner,” Dr. William Schaffner, an infectious-disease expert at Vanderbilt University who was not involved in the research, said in an interview. “This is an extraordinary phenomenon.”…

The new finding also raises the possibility that other outbreaks in the region, assumed to have begun with transmission from animals, may actually have been started by survivors with unrecognized, lingering infections.