There is a decent section on the subject in Jared Diamonds; Guns, Germs, and Steel
Apparently like the Black Plagues in Europe someone always survives and develops resistance to getting the Microbes until time makes the next generations less resistant of or the Disease mutates enough that resistance to it does not matter.

As Dudesky indicated any Disease is easily Transported world wide to those who do not have resistance to it

Another factor is as I read in a Book about the Black Plague in the City of London. A large number of People died because then did not have anyone to care for them let alone even the primitive Medical care of the time.. I am speaking of getting them Food and Water and making sure they were not exposed to the environment adversely.

Despite Disease Man has sort of ruined Natures attempts a balance. There is too many of us to Feed and care for properly and at the same time we are killing of too many other living things supplying ourselves with much more than we need for our survival.

Who ever dies of Ebola will hardly make a ripple in the World Population.

However, no change is on the Horizion.

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