GENEVA -- The World Health Organization declared an end to the deadliest Ebola outbreak ever Thursday after no new cases emerged in Liberia, though health officials warn 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 still is carrying out Ebola tests on dead bodies before burial and remains on the lookout for any suspicious cases.

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.