Liberia, long the hardest-hit country in the Ebola epidemic that has killed nearly 10,000 people in west Africa, "has now gone well over two weeks without a new reported case," said Bruce Aylward, who heads WHO's Ebola response.
Since the outbreak began in December, 2013, 24,282 people in nine countries have been infected with the virus, and 9,976 of them have died, according to the latest figures.
All but 15 of those deaths have happened in the three west African countries ravaged by the outbreak.
The country last week discharged its last confirmed Ebola patient, who tested negative for the virus for the second time on March 3, Aylward told reporters in Geneva.
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That means the country started its 42-day, or two incubation-period, countdown towards being considered Ebola free on March 4.
The outlook was less positive in the other countries affected by the outbreak, Guinea and Sierra Leone, although Aylward highlighted positive signs there too.
"It's going in the right direction," Aylward said.
And in the forest region of Guinea where the outbreak began 15 months ago, no new cases have been reported in the last 10 days, he pointed out.
There is "evidence now that Ebola can be stopped," Aylward said, insisting: "this can be done."
He warned though that if international attention, determination and funding to halt the outbreak wanes, "there is a huge risk of failing to do so."
In both countries some Ebola patients are still not being isolated and treated, while some unsafe burials of the highly contagious bodies are still going on.
Aylward said that it should be possible to halt transmission of the virus completely by the middle of the year, but acknowledged that if community resistance continues "it's a bit of a crapshoot in terms of when transmission will actually stop.