"80% of new patients confirmed are coming outside of known contact lists” ⁠in the heart of the outbreak in Bunia, Ituri province, WHO Emergencies Director Chikwe Ihekweazu said Thursday.

The Ebola outbreak, declared in mid-May, has so far infected nearly 1,800 people and killed 625, according to government data released on ​Thursday. But WHO estimates based on modelling and test positivity rates suggest the outbreak may be between two and four times larger than the number ​of confirmed cases, Ihekweazu said.

However, in areas with fewer cases, like North Kivu province, almost all new cases are coming ​from the contact lists, he added, a sign of some progress.

Strengthening surveillance remains the biggest ​challenge for response measures. Local authorities have begun training 21,000 community health workers to conduct house-to-house visits, identify suspected cases and ​encourage people with symptoms to seek care.