Sex and masturbation have been identified as having more influence in the eradication of the Ebola virus than was previously thought.  Thus, the World Health Organisation,  WHO has advised all male survivors of Ebola to be tested three months after the onset of symptoms and then monthly until they know they have no risk of passing on the virus through their semen.

Bruce Aylward, Head of the WHO's Ebola Response, said on Thursday in Geneva that isolated flare-ups of Ebola may point to a higher risk of transmission via the semen of male survivors than previously thought.

"It's not the sex that is dangerous; it's the semen that is dangerous. How people actually get exposed, in soiled linens or whatever, is not clear.

``Transmission through semen may explain why a few cases continue to occur even though the outbreak has been almost completely eradicated by an intense international effort, recently bolstered by the deployment of a trial vaccine in Guinea and Sierra Leone,’’ he said.

Aylward said the latest flare-up, in a village on the northern border of Sierra Leone, followed the death of a 67-year-old woman late last month, 50 days after the previous confirmed case in the region.

He said transmission chains are considered to have been broken after 42 days with no new infections.

However, Aylward said that sexual transmission was "obviously not a huge risk, because if it were we would have seen a lot more in the areas that were hardest hit at the beginning of this outbreak."

He said this could undermine the hope of ending the outbreak in West Africa by 2015.

A clinician said on condition of anonymity, that a forthcoming study in the New England Journal of Medicine, based on around 200 survivors, found that around half still had traces of the virus in their semen after six months.

"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," clinician said.

The clinician added that the risk might not only be from sex but also from masturbation.