Nine-year-old Nowa Paye is taken to an ambulance after showing signs of the Ebola infection in the village of Freeman Reserve, about 30 miles north of Monrovia, Liberia,Tuesday Sept. 30, 2014.

The World Health Organization declared an end to the deadliest Ebola outbreak ever on Thursday after no new cases emerged in Liberia, though health officials warn that it will be several more months before the world is considered free of the disease that claimed more than 11,300 lives over two years.

Thursday’s success comes after a harrowing toll: Nearly 23,000 children lost at least one parent or caregiver to the disease. Some 17,000 survivors are trying to resume their lives though many battle mysterious, lingering side effects. Studies continue to uncover new information about how long Ebola can last in bodily fluids.

Liberia, which along with Sierra Leone and Guinea was an epicenter of the latest outbreak, was first declared free of the disease last May, but new cases emerged two times — forcing officials there to restart the clock.

“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. “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.”

In Liberia, there was guarded optimism Thursday about reaching the 42-day benchmark with no new cases. The ministry of health is still carrying out Ebola tests on dead bodies before burial, and remains on the lookout for any suspicious cases.

Follay Gallah, an ambulance driver who contracted the disease while intervening in an affected community in 2014, welcomed Thursday’s news but warned: “We could have a recurrence if we don’t do those things that we need to do.”

Ebola is spread through direct contact with the bodily fluids of people who are sick or bodies of the dead. A country is considered free of the disease when it has passed two incubation periods of 21 days without any more cases. However, the most recent flare-up in Liberia confounded scientists as it was not initially clear where the new cases had come from.

WHO now says those cases “are likely the result of the virus persisting in survivors even after recovery.” Of particular concern is the fact it is now known that Ebola is present in the semen of some male survivors up to a year later. The WHO said Thursday that Ebola can “in rare instances be transmitted to intimate partners.”

Before the Ebola epidemic — which is believed to have started in rural Guinea in December 2013 — most of what was known about the disease was limited to studies of much smaller outbreaks in Congo and Uganda. This time, though, the disease made its way to Guinea’s capital, then leaped across borders to Liberia and Sierra Leone. Cases also popped up in Mali, Senegal and Nigeria though transmission chains there were quickly shut down.

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