There are a few things I wonder about Ebola and other especially bad diseases:
1. Wouldn't death rates be a lot lower if it spread to Western countries where we have better medical care (even if there was still no cure)? If we get those fluids and blood transfusions into you better and quicker, it seems like people would do better than 10-50%.
2. Why would any infectious disease evolve to leave a numerous or wide-traveling species like bats or flies alone while infecting a disease with a relatively small and stationary population like Humans? I've always wondered sort of the same thing about the plague. Why not evolve to become the disease that kills billions of rats instead of millions of humans?
In general, I think diseases should find better hosts than humans or use us as natural reservoirs to kill bats, mice, and flies.