An unknown illness has killed over 50 people in northwestern Congo. That's according to doctors on the ground and the World Health Organization on Monday.

The interval between the onset of symptoms and death has been 48 hours in the majority of cases, and “that’s what’s really worrying,” Serge Ngalebato, medical director of Bikoro Hospital, a regional monitoring center, told The Associated Press.

The latest disease outbreak in the Democratic Republic of Congo began on Jan. 21, and 419 cases have been recorded including 53 deaths.

According to the WHO’s Africa office, the first outbreak in the town of Boloko began after three children ate a bat and died within 48 hours following hemorrhagic fever symptoms.