A yet-unidentified illness has killed 53 people and potentially infected hundreds more in the Democratic Republic of the Congo. The World Health Organization is investigating the outbreak.

The WHO Africa office said the quick progression from sickness to death in Boloko is a key concern, along with the high number of deaths in Bomate. Congo’s Ministry of Health said about 80% of the patients share similar symptoms including fever, chills, body aches and diarrhea.

Dozens of people have died and hundreds have become sick after an unknown illness appeared in Congo’s northwestern Equateur province, WHO spokesperson Tarik Jasarevic said Tuesday during a UN briefing.