A day after declaring West Africa free of Ebola, the World Health Organization on Friday confirmed a new case of the disease in Sierra Leone.
Samples taken from the body of a 22-year-old woman who died this week in the northern part of the country tested positive for the virus, local media reported.
The U.N. health agency had warned that more outbreaks were likely in the three countries that have been hardest hit by the two-year epidemic: Liberia, Sierra Leone and Guinea.
The WHO declares a country Ebola-free only after it has gone 42 days without a case, twice the incubation period for the disease.
But research has shown that the virus can persist in survivors for months after recovery, leading some scientists to question the usefulness of the 42-day target.
Sierra Leone’s neighbor, Liberia, experienced two flare-ups after being given the all-clear in May. It was again declared Ebola-free Thursday.
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Sierra Leone first achieved the milestone Nov. 7. The new case emerged during a 90-day period of heightened surveillance, and the government responded quickly, the WHO said.
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Sanitized gloves and boots hang to dry as a burial team collects Ebola victims from a Ministry of Health treatment center for cremation on Oct. 2 in Monrovia, Liberia. (John Moore / Getty Images)
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Medical personnel at the Ebola treatment center at Island hospital in Monrovia, Liberia, disinfect people who had brought patients suspected of having the Ebola virus on Oct. 2. (Pascal Guyot / AFP/Getty Images)
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A man transports a possible Ebola victim in a wheelbarrow on Oct. 2 at a treatment center at Island hospital in Monrovia, Liberia. (Pascal Guyot / AFP/Getty Images)
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A health worker attends to a girl at the Doctors Without Borders Ebola treatment unit in Monrovia, Liberia. (Pascal Guyot / AFP/Getty Images)
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Markers for the graves of Ebola victims await placement in Bong County, Liberia. (Robyn Dixon / Los Angeles Times)
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Nurses receive training from the World Health organization on how to use equipment to protect themselves from Ebola in the Sierra Leone capital, Freetown, on Sept. 18. (Michael Duff / Associated Press)
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A burial team, clad in protective gear, carries the body of a child at the International Medical Corps Ebola treatment unit in Bong County, Liberia. The girl, who watched her mother grow sick and die of the virus, died alone and terrified, workers said. (Robyn Dixon / Los Angeles Times)
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Medical staff members in protective suits work in the high-risk Ebola area of the ELWA Hospital in Monrovia, Liberia. (Dominque Faget / AFP/Getty Images)
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Traffic is stopped at a Guinean security force roadblock to screen people for symptoms of Ebola near the town of Forecariah, Guinea. (Youssouf Bah / Associated Press)
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A health worker looking for signs of Ebola checks a man’s temperature at a roadblock run by Guinean security forces outside the town of Forecariah, Guinea. (Youssouf Bah / Associated Press)
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A vehicle loaded with household items pauses at the border town of Diaobe in Senegal’s southern border region as people prepare to drive into Guinea. The Ebola epidemic is spreading in West Africa, spurring people to flee. (Seyllou / AFP/Getty Images)
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A worker in a protective suit empties possibly Ebola-contaminated waste in a high-risk quarantine zone of the John F. Kennedy Memorial Medical Center in Monrovia, Liberia. (Dominque Faget / AFP/Getty Images)
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A man who was showing symptoms of possible Ebola listens as UNICEF health workers speak about Ebola prevention in New Kru Town, Liberia. (John Moore / Getty Images)
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A resident checks on a very sick Saah Exco, 10, in a Monrovia alley. The Liberian boy was one of a number of patients pulled from a holding center for suspected Ebola patients when the facility was overrun by a mob. A local clinic refused to treat him, residents said. (John Moore / Getty Images)
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Workers from the Liberian Ministry of Health disinfect themselves before burning the bodies of Ebola victims at a crematorium in Marshall, Liberia. (John Moore / Getty Images)
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A quarantine officer checks the body temperature of a passenger as a precaution against the Ebola virus at the Incheon International Airport in South Korea. South Korea has stepped up monitoring of citizens returning from West Africa. (Choe Jae-koo / Associated Press)
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Ebola patient Nancy Writebol is wheeled into Emory University Hospital in Atlanta. (John Spink /Atlanta Journal-Constitution / MCT)
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A Nigerian crew waits to screen passengers for Ebola symptoms at the arrival hall of the international airport in Lagos. (Sunday Alamba / Associated Press)
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A 10-year-old boy is showered after being taken out of quarantine following his mother’s death from the Ebola virus, at the Samaritan’s Purse Ebola treatment center in Monrovia, Liberia. (Zoom Dosso / AFP/Getty Images)
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A nurse with Doctors Without Borders examines a patient at a center in Gueckedou, Guinea. (Seyllou / AFP/Getty Images)
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Social commentator Alfred Sirleaf, center rear, discusses the virus in Monrovia, Liberia. (Jonathan Paye-Layleh / Associated Press)
Local authorities and their international partners were investigating the origin of the case, identifying those who came into contact with the woman and taking measures to prevent further spread of the disease.
The epidemic that began in Guinea in December 2013 has infected more than 28,500 people and killed 11,300, according to the latest figures from the WHO.
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“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, the WHO’s special representative for the Ebola response, said Thursday. “We still anticipate more flare-ups and must be prepared for them.â€
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