The Ebola virus can linger in a man's semen for nine months or longer, researchers reported Wednesday, and in at least one case they've confirmed that a survivor infected someone else after seemingly being cleared of the virus.
The researchers don't think it's especially common, and they cannot say if the virus remains infectious in semen after months and months.
"We believe it's rare," said Dr. Barbara Knust, who helped lead CDC's Ebola response team.
But it's a troubling reminder that the epidemic that killed more than 11,000 people in Guinea, Sierra Leone and Liberia could have some very long-term effects.
One report in the New England Journal of Medicine confirms what experts had feared - a 44-year-old woman who died in Liberia last March was infected by her fiancé, who had survived Ebola.
Researchers had suspected this might happen, but this is the first proof. Gustavo Palacios of the United States Army Medical Research Institute of Infectious Diseases and his colleagues genetically matched the virus that killed the woman to samples taken from her fiancé, who had been declared free of the virus for months.
After that, the World Health Organization warned that survivors should abstain from sex or use condoms for 90 days.
But the second report shows the virus can remain in a man's semen for months longer than previously believed.
The team from Sierra Leone's government, WHO and the Centers for Disease Control and Prevention recruited 93 male Ebola survivors who agreed to be regularly tested after they were cleared of the virus.
The first nine men, tested three months after they first got sick, all still had evidence of the virus in their semen, the team reported in the New England Journal of Medicine.
Another 65 percent of those tested four to six months later had genetic evidence of Ebola in their semen and 26 percent of those tested seven to nine months later did.
"Survivors should be abstaining from sex or using condoms for a much longer period of time than they originally had been told," Knust told NBC News.
Sex is almost certainly not an important way to spread Ebola, which mostly infects people through bodily fluids spread when patients are extremely ill.
"If sexual transmission from survivors were an important means of disease propagation, we would have seen a number of cases by now," Armand Sprecher of Doctors Without Borders in Brussels wrote in a commentary.
However, the Ebola epidemic that appears to be winding down has affected many, many more people than any previous outbreak. More than 28,000 people were infected and more than 11,000 died.
"There are a lot of survivors in West Africa," Knust said. Researchers had suspected perhaps 20 cases in years past, in sporadic outbreaks across central Africa, when sexual transmission may have re-introduced the virus long after one outbreak. But it had been impossible to prove.
WHO has known that Ebola survivors can have long-term effects from the virus.
Recovered patients complain of severe joint pain, and live virus was found in the eye of Dr. Ian Crozier, an American doctor who reported eye problems after he recovered from Ebola infection.
And now British nurse Pauline Cafferkey, who recovered from an Ebola infection last January, is in critical condition in a hospital in Scotland with what doctors say are complications from her infection.
It doesn't appear that the full infection has returned in Cafferkey, Knust said. Instead, it appears that some virus may have remained in the spinal fluid or elsewhere in the body to cause severe symptoms. "It's possible that the virus could persist there," she said.
It's clear that in these cases people do not have the virus in their blood and they don't appear to be able to infect anyone else. In the case of semen, it's not clear if the virus is even intact. It doesn't cause any symptoms for the men. The test that detects it finds two pieces of genetic material, not whole virus.
"We don't have a great understanding of what exactly is going on in the male reproductive tract," Knust said.
"The testicles in particular are an area where the immune system does not have nearly as strong a presence as in other parts of the body."
The places where the virus can hide out in the body are called immune privileged sites. They're protected from the immune system cells that normally destroy viruses. The testes are one place; so are the eyes, parts of the joints and the brain.
Knust said CDC experts are now trying to grow samples from the semen of the men in Sierra Leone to see if the virus is whole and can infect others. That work's being done in the most secure, biosafety level 4 labs.
The WHO and CDC say it's important to stress that Ebola survivors shouldn't be shunned.
"These survivors have gone through so much and on top of having a life-threatening illness, many of them have lost their families and their jobs," she said.
"The community has also rejected them in many different ways as well."
There was one piece of good news in the Ebola epidemic Wednesday. WHO says no new cases have been reported for two weeks. A country is declared Ebola free if 42 days pass without a new case.