MONROVIA, Liberia (AP) — The World Health Organization says Liberia has been declared free of Ebola transmissions and will be under heightened surveillance for 90 days.

The organization said Thursday that 42 days - twice the maximum incubation period - have passed since the second negative test of a laboratory-confirmed case.

More than 4,800 people died in Liberia of Ebola before the country was first declared free from transmissions on May 9. Then a post-mortem sample from a 17-year-old who died on June 28 came back positive for Ebola. Five more cases were confirmed, including one more death from Ebola.

The worst Ebola outbreak in history has killed more than 11,300 people overall, hitting primarily Guinea, Sierra Leone and Liberia.