...The more people who are infected with the current strain of Ebola, the more opportunities it has to mutate into a form that is infectious in the air. This article estimates that 1.2 million people will die if that happens. I strongly suspect that the number of deaths will be far greater. Currently, there are over 4,200 cases of infections, and over 2,400 known deaths.
UPDATE: And according to this article, we could get to one million patients in 18 weeks or less, even if it doesn't go airborne.