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.