[lotsofmangos]

According to More or Less, 90% was the mortality rate of one particular historical outbreak.

http://www.bbc.co.uk/podcasts/series/moreorless

http://downloads.bbc.co.uk/podcasts/radio4/moreorless/moreor...

edit - It also says that 52% is the current figure on this outbreak and that if you count up all cases, then Ebola has a mortality rate of 60-65%.