It can take between two and 21 days for someone infected with the virus to begin showing symptoms, and it's been difficult for officials to convince some patients to stay in quarantine for that length of time.
The WHO says every time a new clinic opens in Liberia, a flood of previously unreported Ebola patients arrives, suggesting there is a large population of undiagnosed Ebola cases in the country.
At the same time, many clinics and treatment centres have closed their doors because doctors and patients are too afraid to expose themselves to the virus, the WHO says.
There is currently no WHO-approved vaccine for the Ebola virus, although some experimental vaccines have been produced and tested in an effort to stem the outbreak. However, the vaccines have already run out.
Patients infected with Ebola require intensive supportive care, which can be taxing on hospital resources and poses a risk to healthcare workers who can be easily infected. There is no specific treatment for Ebola patients, other than to keep them constantly hydrated.