Health officials in Sierra Leone said on Thursday a woman who died in a northern village tested positive for Ebola, as the deadly disease keeps mysteriously flaring up.

Earlier on Thursday, WHO declared Liberia free of the disease after the country passed a 42 day period - or two incubation periods - without registering a new case. It had discharged its last two patients with the disease on December 3, 2015.

The two year epidemic infected almost 29,000 people and killed some 11,000, devastating the economies and health systems of the hardest hit countries: Liberia, Sierra Leone and Guinea.

Even as WHO declared West Africa free of the disease on Thursday, health officials said that it was likely for flare-ups to persist although they would become less frequent over time.

"While this is an important milestone and a very important step forward, we have to say that the job is still not done," said Rick Brennan, WHO director of emergency risk assessment and humanitarian response, at a news conference in Geneva on Thursday. "That's because there is still ongoing risk of re-emergence of the disease because of persistence of the virus in a proportion of survivors."

The virus and residual symptoms have mysteriously remained in some survivors even after they have been released from the hospital. Alarmingly for health officials, some research suggests the virus can stay in men's semen for up to a year.

Liberia was declared free of the virus twice - in May and September - only for small clusters of the virus to pop up again, before being announced clear again on Thursday. WHO declared Guinea, where the outbreak began in December 2013, to be free of Ebola last month.

cw/rc (AFP, dpa)