Although researchers have known since 1999 that traces of the Ebola virus could remain in semen for months, two papers published in The New England Journal of Medicine today offer more detail about the frightening possibility that survivors of an infection could rekindle outbreaks. One study focuses on nearly 100 men in Sierra Leone who survived the dreaded viral illness, whereas the second one documents a clear case of sexual transmission of Ebola virus.

In the Sierra Leone study, researchers found Ebola viral RNA in semen samples from almost half the 93 men they tested.  The likelihood of finding viral RNA declined as time from disease onset increased: All nine men who were tested 2 to 3 months after they fell ill had evidence of Ebola RNA in their semen, but the researchers  found it in only 26 of 40 men whose infections had started 4 to 6 months earlier and in 11 of 43 men whose infections had started 7 to 9 months earlier. The result from one Ebola patient tested 10 months after disease onset was indeterminate.

Detecting viral RNA does not mean that these survivors harbor a virus that’s capable of establishing an infection in a sexual partner. “We do not yet have sufficient information to assess the risk of transmission through sexual intercourse, oral sex, or other sex acts from men with viable virus in their semen,” the authors from Sierra Leone and the World Health Organization (WHO) wrote. Scientists at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta are attempting to isolate virus from the semen samples, says Nathalie Broutet, an infectious disease epidemiologist at WHO in Geneva and one of the study’s authors.

In the second paper, researchers from Liberia and the United States present the best evidence yet of sexual transmission. A 44-year-old woman from Montserrado County was diagnosed with Ebola on 20 March and died a week later. The country had not had a case of Ebola in the previous 30 days and there was no obvious source of infection, but the patient reported having had unprotected vaginal intercourse with an Ebola survivor on 7 March. The man had contracted Ebola in September 2014 and left the Ebola treatment unit after testing negative for the virus in early October. A semen sample from the man, taken in March 2015, tested positive for Ebola, and genetic analysis of the woman’s virus showed that it was distinct from the most recent clusters in Liberia and neighboring countries. Importantly, her virus was all but identical to the one isolated from the survivor: Only one base pair differed between the two genomes. The “unique gene signatures in the sample obtained from the semen of the survivor and the deceased woman I think really provides conclusive evidence,” says study co-author Vincent Munster, a virologist at the U.S. National Institute of Allergy and Infectious Diseases in Bethesda, Maryland.

CDC’s Broutet says sexual transmission is suspected in about 20 West African cases. The WHO changed its advice  to survivors in May this year after evidence showed that the virus could persist much longer in semen than previously thought. The guidelines now advise survivors to abstain from sex or use a condom for 6 months or until their semen tests negative. Given that there are thousands of male survivors who could spread the virus through sex, “the chances of seeing sporadic cases igniting small outbreaks is very real,” says Jonathan Ball, a virologist at the University of Nottingham, in the United Kingdom.

The papers come amid news that Pauline Cafferkey, a British nurse who fell ill with Ebola in December 2014 and survived, was in critical condition at Royal Free Hospital in London and being treated “for Ebola” after having apparently suffered a relapse. All of this shows that the virus has the potential to surprise scientists, Munster says. “An outbreak of this magnitude has never happened before with Ebola virus, so I think we need to realize that data gathered from the previous outbreaks might not be sufficient.”

Survivors have already endured a painful illness and often lost loved ones, Armand Sprecher of Doctors Without Borders in Brussels cautions in an editorial that accompanies the two papers. “If they are then treated as pariahs and threats, we add a terrible unkindness on top of their suffering,” Sprecher writes. “They should be treated with all the compassion we can muster.”

WHO today released a situation report that said for the second straight week, no new cases of Ebola virus have been confirmed in any West African country.