A corpse has tested positive for Ebola in Sierra Leone, an official said Friday – just one day after the World Health Organization declared the outbreak over in West Africa.

Francis Langoba Kelly, spokesman for Sierra Leone’s Office of National Security, told a local radio program that tests on a 22-year-old woman who died this month were positive for the virus, according to the Associated Press.

The WHO confirmed the new case, saying in a news release that its discovery reflects “the ongoing risk of new flare-ups of the virus in affected countries.”

“The Sierra Leone government acted rapidly to respond to this new case,” the U.N. health agency said.

“Through the country’s new emergency operations center, a joint team of local authorities, WHO and partners are investigating the origin of the case, identifying contacts and initiating control measures to prevent further transmission,” the agency said.

On Thursday, the WHO declared that the Ebola outbreak in West Africa – which left more than 11,300 dead and triggered a worldwide epidemic of fear and anxiety – had officially ended.

The WHO made that declaration 42 days after Liberia’s last Ebola patient tested negative for the disease twice – the traditional marker of an epidemic’s conclusion.

The two other worst-affected countries, Sierra Leone and Guinea, were declared Ebola-free last year.

Still, the organization warned Thursday that “more flare-ups are expected and that strong surveillance and response systems will be critical in the months to come.”