FREETOWN - Sierra Leone's leading doctor died of Ebola on Thursday, hours after the arrival in the country of an experimental drug that could have been used to treat him, the government's chief medical officer said.Victor Willoughby was diagnosed with Ebola last week after he treated a man with organ-related problems. The patient, a senior banker, was later diagnosed with Ebola and has since died.The drug, ZMab, was transported in frozen form on a Brussels Airlines flight that arrived overnight. Before it could thaw, Willoughby's condition deteriorated, said chief medical officer Brima Kargbo.His death brings to 12 the number of Sierra Leone doctors to have contracted the virus. Eleven have died. In all, 142 health workers have been infected with the disease in the West African country and 109 have died, according to World Health Organization figures.Sierra Leone, neighboring Guinea and Liberia are at the heart of the world's worst recorded outbreak of Ebola. Rates of infection are rising fastest in Sierra Leone, which now accounts for more than half of the 18,603 confirmed cases of the virus.The overall death toll from the epidemic has risen to 6,915 as of December 14, the WHO said on Wednesday, adding that the increase in cases in Sierra Leone appeared to have slowed.Kargbo said Willoughby's death was one of the most tragic to hit the country since the passing, in July, of its only virologist and Ebola specialist, Dr Shek Humar Khan.