Some men still have fragments of Ebola virus in their semen nine months after first showing symptoms of the virus, according to a small, preliminary study published today in the New England Journal of Medicine.

Scientists often test for diseases by sequencing genes from a sample — usually DNA, which encodes genetic information, or its cousin RNA, which controls how the genes express themselves. Here, the scientists found RNA from Ebola in the semen of some men long after they had recovered from their infection. Out of 40 men who were tested in Sierra Leone, 26 produced semen that contained Ebola RNA six months after the onset of the illness, the report shows. Nine months after showing symptoms, that number appears to drop; a total of 11 out of 43 men — about a quarter — produced semen that tested positive for the virus.

Ebola survivors face "recognized health complications."

That means that in some cases, genetic fragments of Ebola can persist in semen for long periods. That’s important for controlling the epidemic, which infected 300 to 400 people a week in Liberia alone at its peak. In the last three weeks, only four people were infected, all of them in Guinea — and while that's substantially lower, it's still not zero. To end the outbreak, which began in 2014, understanding exactly how long someone remains contagious is important, and today’s finding may suggest survivors’ sperm may be infectious — or that there are reservoirs of the virus in the body somewhere.

"Ebola survivors face an increasing number of recognized health complications," CDC director Tom Frieden, said in a statement. "This study provides important new information about the persistence of Ebola virus in semen and helps us make recommendations to survivors and their loved ones to help them stay healthy."

Researchers don't know why Ebola RNA persists in semen, or even if semen can spread the virus. A second study, based on a single case and published today in the NEJM, suggests that the sexual transmission of Ebola might be possible. A woman in Liberia may have been infected after she had unprotected sex with a male Ebola survivor, the study states; the survivor had been infected six months prior. But the researchers behind that study can't say for sure if that's what happened. Larger studies are needed to determine if this kind of transmission really can take place, they say. The US Centers for Disease Control and Prevention are now conducting additional tests to see if the virus contained in semen is live and infectious.

Abstain from sex or use condoms until their semen tests negative twice

This isn't the first time that researchers have reported the persistence of Ebola in humans. One doctor at Emory University who was infected with Ebola returned to the hospital less than two months later because of intense pain in his eye. The doctors were shocked to the find that even though he had recovered, the inside of the physician's eye was full of Ebola. Eventually, the doctor recovered his eyesight — but this unpleasant surprise suggests that the body might be able to harbor the virus for longer than physicians once thought.

Until scientists know more, the CDC recommends that men who have survived the virus should abstain from sex or use condoms until their semen tests negative twice. People should also wash their hands after coming in physical contact with the bodily fluid. (Ebola doesn't spread through the air; only contact with bodily fluids that contain the live virus can spread infection.) All the men who participated in the first study were given condoms and provided with counseling. Since the beginning of the outbreak in West Africa, about 28,000 people have been infected; 11,000 have died.

"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 statement. Survivors "need continued, substantial support for the next 6 to 12 months to meet these challenges, and to ensure their partners are not exposed to potential virus."