The World Health Organization says the number of Ebola cases reported in Guinea and Sierra Leone last week dropped to 18, the lowest total this year. 

In an update published this week, the U.N. health agency said the situation was "encouraging" but noted officials are still unable to track exactly where the virus is spreading. In Guinea, five of the country's nine new cases were detected only after the patients died; none had sought treatment in a hospital. In Sierra Leone, only two of the nine cases were identified as contacts of previous Ebola patients.

In Liberia, the outbreak will be declared over on May 9 unless any new cases are announced.

To date, WHO estimates Ebola has killed more than 11,000 people, the vast majority in West Africa.