The World Health Organization announced Thursday that the Ebola epidemic in West Africa is over — for now.

For the first time since the outbreak began in December 2013, all three of the hardest-hit West African nations — Liberia, Guinea and Sierra Leone — have had zero reported cases of Ebola for 42 days in a row. That's equal to two full incubation cycles of the virus.

The most recent outbreak was in Liberia, which had previously been declared Ebola-free in May and in September last year. Each time, at least one new case was later discovered. The last confirmed patient has now tested negative for the disease two times, the WHO says.

The WHO emphasized that all three countries "remain at high risk" of small-scale re-occurrences of the disease.

As NPR's Goats and Soda blog reported in November, when the most recent flare-up in Liberia began, researchers are continuing to learn about how survivors can and can't pass the disease to others long after the standard incubation cycle is over.

The Ebola virus can remain in semen for 9 months, for example, far longer than previously understood, raising the risk of sexual transmission of the disease once it has otherwise stopped spreading through a population.

As NPR's Nurith Aizenman reported onMorning Edition Thursday, some survivors of Ebola also wind up with the disease trapped inside their body — in an eye infection, for instance — and can get sick again from the same virus.

"Then there's the risk of a totally new chain of infection," Nurith says, noting that Ebola could be reintroduced to West Africa from wild animals, the same way the recent epidemic was believed to have started.

With the risks and uncertainties in mind, WHO says the news that West Africa is Ebola-free is cause to celebrate, but not to stop working.

"We are now at a critical period in the Ebola epidemic as we move from managing cases and patients to managing the residual risk of new infections," Dr. Bruce Aylward, WHO's special representative for the Ebola response, said in a statement.

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