The World Health Organization has declared Liberia free from Ebola, ending the worst outbreak of the disease in West Africa. The country joined Sierra Leone and Guinea as Ebola-free, reported AP. However, United Nations Secretary-General Ban Ki-moon has warned against the possibility of flare-ups of the virus in West Africa over the year. Liberia was declared Ebola-free twice before, only for another case to be reported.

The latest announcement of the “end of active transmission” was made 42 days after no new case of the disease was reported in the country, BBC reported. The WHO declares a country free of human-to-human transmission after two 21-day incubation periods are completed since the last known case is tested negative for a second time. Guinea was declared free of the epidemic last month.

WHO chief Margaret Chan said the next three months will be “the most critical” for the three west African nations, which accounted for nearly all the deaths caused by the outbreak. The disease has claimed as many as 11,315 lives since December 2013, of which 4,809 were in Liberia.