GENEVA (Reuters) - Sex could keep the Ebola epidemic alive even after the World Health Organization (WHO) declares an area free of the disease, one of the discoverers of the deadly virus said on Tuesday.

The WHO is hoping to announce later this week that Nigeria and Senegal are free of Ebola after 42 days with no infections -- the standard period for declaring an outbreak over, twice the maximum 21-day incubation period of the virus.

However, it appears the disease can last much longer in semen.

"In a convalescent male, the virus can persist in semen for at least 70 days; one study suggests persistence for more than 90 days," the WHO said in an information note on Monday.

"Certainly, the advice has to be for survivors to use a condom, to not have unprotected sex, for 90 days," said Peter Piot, a professor at the London School of Hygiene and Tropical Medicine and a discoverer of Ebola in 1976.

"If we would apply the rule for double the time, that would be 180 days -- six months. I think it (90 days) is probably a compromise, for practicality," he told a news conference in Geneva.

Ebola spreads via bodily fluids such as blood and saliva, but it has also been detected in breast milk and urine, as well as semen, the WHO says. The whole live virus has never been isolated from sweat, however.

More than 3,400 people are already known to have died in the world's worst Ebola outbreak on record, the vast majority of them in three West African countries: Guinea, Liberia and Sierra Leone.

(Reporting by Tom Miles; Editing by Kevin Liffey)