The World Health Organization says officials are now focused on ending the biggest-ever Ebola outbreak rather than just slowing the virus' spread.

In an update published Thursday, the U.N. health agency said the three most affected countries — Guinea, Sierra Leone and Liberia — reported fewer than 100 cases in the past week, for the first time since June.

Still, WHO also reported that only 30 percent of new cases in Guinea are known contacts of Ebola patients, meaning officials have no idea how most patients are getting infected. In Liberia, that figure is 50 percent, while no data are available for Sierra Leone. The death rate for hospitalized patients is about 60 percent.

The outbreak is believed to have killed more than 8,000 people since it started in March.