In previous cases, the cases are linked to a known disease, but a lack of available testing leads to lack of certainty. For example, there was a separate report of an "unknown disease" in December of last year in Congo that was later attributed to illnesses from malaria and respiratory illnesses.

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.