The Ebola epidemic affecting West Africa is predicted to last a further 12 to 18 months, according to U.S. scientists. Epidemiologists have been creating computer models of the Ebola epidemic for the National Institutes of Health and the Defense Department. The model they have created is a far less optimistic estimate than that of the World Health Organization (WHO), which last month said it hoped to contain the outbreak within nine months and 20,000 total cases. Scroll down for video Spreading: A child looks at a man  suspected of suffering from the Ebola virus, while holding his hand over his nose,  in a main street and busy part in Monrovia, Liberia, Friday, September 12
@highlight
U.S. scientists say the Ebola crisis is worsening
@highlight
They predict the virus will rage for another 12 to 18 months
@highlight
As of September 7, there had been 4,366 Ebola cases including 2,218 deaths, more than half of them in Liberia
@highlight
The most recent figures from Liberia reported 400 new cases as of September 7 - almost double the number reported the previous week
International plea: The @placeholder said on Friday at least 500 foreign experts are needed to help deal with Ebola in West Africa