The body of a 17-year-old Liberian boy tested positive for Ebola on Tuesday, 52 days after the country was declared free of the disease by the World Health Organization.

“Liberia has got a re-infection of Ebola,” deputy health minister and head of Liberia’s Ebola response team, Tolbert Nyenswah, told the Associated Press, adding that the boy died at his home and was buried safely to avoid spreading the virus.

Nyenswah said teams are investigating how the boy became infected. The area is not near the borders with Sierra Leone and Guinea, which still have Ebola cases.

There have been more than 27,000 reported confirmed, probable, and suspected cases of the Ebola virus in the three countries, with more than 11,000 reported deaths, according to the WHO’s latest situational report. An estimated 20 to 27 people are still being infected each week in Sierra Leone and Guinea.

At least four cases of Ebola have recently been detected in the northern Guinea prefecture of Boke, which borders the nation of Guinea-Bissau. That country has not yet had a case of Ebola.

Because of fears that the disease may hop yet another border, the WHO has deployed two epidemiologists and two community engagement experts to Guinea-Bissau.

Liberia was declared free of Ebola on May 9 after it went 42 days without a new case and nearly 5,000 people had died of the disease. The actual death toll is likely to be higher than that, since many people died at home and were never counted as official victims.

Now the 42-day clock starts again.

Some information in this report is provided by The Associated Press.