A new study by the World Health Organization has found that the Ebola virus can live in the semen of survivors for at least nine months, dramatically increasing the risk of sexual transmission of the disease in west Africa, where an outbreak that began in March 2014 has not yet been fully contained.

The study, published in the New England Journal of Medicine on Wednesday, studied male Ebola survivors, regularly testing bodily fluids for the presence of the virus. After three months, all of 93 men monitored in Freetown, Sierra Leone tested positive for Ebola. More than half did so by six months’ time, but up to 25% of respondents tested positive for Ebola after nine months, indicating the potential that Ebola survivors can spread the disease to their sexual partners as long as nine months after recovery.

While the research confirms the presence of the virus, it does not definitely prove that Ebola survivors are contagious and that Ebola remains a sexually transmitted disease nearly a year after a survivor fully recovers. “We think there is a potential risk of exposure but we cannot determine that with 100 percent certainty right now,” Dr. Nathalie Broutet, one of the study’s authors, said. The study also did not explain what made semen so particularly prone to keeping the Ebola virus alive as opposed to other bodily fluids, like blood. Contact with the blood of an Ebola patient is the fastest and easiest way to contract the disease.

“These results come at a critically important time, reminding us that while Ebola case numbers continue to plummet, Ebola survivors and their families continue to struggle with the effects of the disease,” Bruce Aylward, WHO Director-General’s Special Representative on the Ebola Response, said in a statement, adding that the study definitively proves that Ebola survivors “need continued, substantial support for the next six to 12 months to meet these challenges and to ensure their partners are not exposed to potential virus.”

A first step to keeping the virus from continuing to spread would be to encourage Ebola survivors to practice safe sex for many months following recovery as scientists figure out the precise survival rates of Ebola in human semen. Dr. Francis Moses, a district medical officer in northern Sierra Leone, tells CBS News that keeping Sierra Leone’s Ebola survivors from endangering their sexual partners has been an uphill battle. “The abstinence thing isn’t working,” he said, citing the number of pregnant partners of Ebola survivors he has treated during his time in Sierra Leone.

While the rates of Ebola diagnoses have slowed significantly since the outbreak peaked in mid-2014, cases continue to surface, particularly in Sierra Leone. In many of the most recent cases, sexual contact with an Ebola survivor has been the suspected mode of contraction.

“The old advice of three months is no longer good. The number of people with persistent virus in their semen is much greater than expected,” a doctor studying the sexual transmission of Ebola told Reuters in September. The doctor emphasized that semen being the mode of transmission, and not simply any sexual contact, made the virus even more dangerous, as activities outside of intercourse, like masturbation, could lead to the unknowing transmission of Ebola.

At the time, the World Health Organization called the status of Ebola survivors “an emergency within an emergency.”

Ebola survivors often return from the hospital to find that they have lost their jobs, their significant others want nothing to do with them, and only their families will speak with them at all. “People looked at me like I’d come back from the dead, like I was a zombie,” Jamila, a teacher in Guinea, said of her experience. Liberian Dr. Melvin Korkor described the social stigma as “worse than the fever.”

In addition to the threat of continuing to spread Ebola, survivors often report “joint pain, headaches or muscle pain… sleeplessness and visual problems.”

Eye problems have presented scientists with a particularly strange complication of Ebola: the possibility that the virus will live on in the eyes of survivors. Dr. Ian Crozier, who worked as a volunteer in west Africa, contracted and survived Ebola. Two months after being apparently cured, doctors found the virus living in the fluid inside his eyeballs, but not in his tears or the surface of his eye. He began to experience loss of sight and extreme pain in his eye.

In Guinea, where the virus started, its mysteries continue to baffle doctors. Researchers have discovered a group of women in Guéckédou, where the outbreak began, who have developed extreme immunological resistance to Ebola despite never showing symptoms of having the virus inside them. Most were forced to care for relatives who died of the disease and were exposed to bodily fluids carrying Ebola, but never contracted the disease.