Thomas House has just published an analysis of previous Ebola outbreaks in eLife.

This analysis showed a fast rate of new outbreaks, and significant variability in case fatality ratios between strains. Initially, the results on transmissibility of Ebola suggested that the current outbreak could be viewed as a rare event within the pattern of previous epidemics.

As the numbers infected continue to rise, this becomes less and less likely, and it seems that something more fundamental has changed - see the accompanying press resease for more comment.