Health officials are narrowing in on the cause of dozens of unexplained deaths in the Democratic Republic of Congo. The Africa Centers for Disease Control and Prevention said Thursday that a severe form of malaria is likely responsible, based on lab tests so far.

The illness was considered a mystery for weeks. It has killed at least 37 people and sickened 592, according to the Africa CDC. Local authorities previously told Reuters that the death toll exceeded 140.

Health officials in Africa…