Ebola isn’t finished yet.

Liberia has long since celebrated a momentous milestone, being declared Ebola-free, with no new cases in months. But for the second week in a row, the number of new cases is up in both Sierra Leone and Guinea.

There was a time in May when the number of new cases in those two countries was tantalizingly close to zero. Then, suddenly, came an alarming spike in the two countries, which were ravaged by the virus during the deadliest Ebola outbreak in history.

Now, week after week, up to a few dozen new people are appearing on the World Health Organization’s weekly report on the situation in West Africa. And public health officials are racing to find out what’s driving these pockets of Ebola transmission.

Sierra Leone’s chief medical officer, Brima Kargbo, diagnosed part of the problem as an inability to convince some people to not seek treatment from local healers.