Another factor - when it comes to the varying CFR from one outbreak to the next - may be the different strains of the disease. Of the five known ebola strains, the "Zaire" and "Sudan" strains have been responsible for most deaths. The Zaire strain's average fatality rate is 79% and the Sudan strain's is 54% - research on the current outbreak, in Guinea, suggests that it is caused by the Zaire strain.