“The evidence has grown over the years that the virus can be found in semen beyond three, to even nine months, or even two years in some cases,” said Ngoy Nsenga, the WHO official. “Part of our counseling of survivors is to teach them that sexual activity does carry a risk of transmission, and even to conduct testing on their semen to ascertain whether they are free of the virus.”

Studies conducted after the world’s worst Ebola outbreak, which spread widely across Guinea, Liberia and Sierra Leone between 2014 and 2016 and killed more than 11,000, gave early indications that the Ebola virus could remain active in various bodily fluids besides blood, which is the most common vector for its transmission.

The deceased woman’s husband was discharged from an Ebola treatment center in the city of Butembo less than a year ago. Regardless of whether the new case is found to be linked to the last outbreak, officials will consider this to be a new outbreak, the 12th in Congo since Ebola was discovered in 1976.

“Technically, because transmission can start anew from this case, it will be considered its own outbreak,” said Eta Ngole, deputy programs director for the International Medical Corps, who spoke by phone from Butembo, a city of nearly 1 million people that was the epicenter of the previous outbreak. It is also where the woman with Ebola died last week.

Ngole said the deceased’s husband’s semen had tested negative for Ebola after he was discharged from the treatment center but that his organization did not do the testing and that tests have in the past returned false negatives. He said only the samples being studied by Congo’s health ministry would be able to definitively prove the link between his illness and his wife’s.

In past cases, Ebola has been found months later in survivors’ eye fluids, Ngole said, and the same was true for vaginal fluids, but it appears to remain the longest in semen, according to Nsenga.

The previous outbreak ravaged eastern Congo’s North Kivu and Ituri provinces for nearly two years, killing around 2,300 people and compounding stresses on a region beset by decades of conflict over resources and ethnic rivalries.

The challenges led the Ebola treatment center in Butembo, where the deceased woman’s husband was discharged last year, to fall into disrepair.

Ngole and other first responders are part of a team rehabilitating the center.

“The tents are now in poor shape and can’t be used to isolate cases, so we are hoping that this time we can rebuild with semi-permanent material,” he said.

Nsenga said Congo’s health ministry was leading an effort to trace around 70 contacts of the deceased woman. Concerns about spread are lessened this time around, he said, because many in and around Butembo were vaccinated during the last outbreak there. Two highly effective Ebola vaccines were approved and distributed over the past year.

“The patient had been handled at several health facilities and was transported by motorbike,” he said. “But maybe some of those contacts may have been vaccinated. Just under 304,000 were vaccinated over course of last outbreak.”

Approximately 8,000 doses of the vaccine were in Congo at the moment, Nsenga said. Some 2,000 were in Equateur province, where another outbreak was recently declared over, and 6,000 were in the capital, Kinshasa. Nsenga said they were being shipped to Butembo as soon as Wednesday.