U.S marines arrival at the Roberts International airport in Monrovia, Liberia, Thursday, Oct. 9, 2014. Six U.S. military planes arrived Thursday at the epicenter of the Ebola crisis, carrying more aid and American Marines into Liberia, the country hardest hit by the deadly disease that has devastated West Africa and stirred anxiety across a fearful world. At a World Bank meeting in Washington, the presidents of several West African countries struggling with Ebola pleaded for help, with one calling the epidemic "a tragedy unforeseen in modern times." (AP Photo/Abbas Dulleh)

U.S marines arrival at the Roberts International airport in Monrovia, Liberia, Thursday, Oct. 9, 2014. Six U.S. military planes arrived Thursday at the epicenter of the Ebola crisis, carrying more aid and American Marines into Liberia, the country hardest hit by the deadly disease that has devastated West Africa and stirred anxiety across a fearful world. At a World Bank meeting in Washington, the presidents of several West African countries struggling with Ebola pleaded for help, with one calling the epidemic "a tragedy unforeseen in modern times." (AP Photo/Abbas Dulleh)