DAKAR (Reuters) - More than 1,900 people have died in the world’s worst outbreak of Ebola in West Africa, the head of the World Health Organization said on Wednesday, marking a major acceleration in fatalities from just over 1,500 last week.

Margaret Chan told a news conference in Washington that 3,500 confirmed or probable cases of Ebola have been reported in Guinea, Sierra Leone and Liberia as of this week. The death toll from this outbreak is now higher than in all the previous epidemics since the disease was first detected in 1976.