The longer Ebola continues to grow in Africa the more likely it will go to the middle east, Europe and beyond

Results: We model the short-term growth rate of the disease in the affected West African countries and estimate the basic reproductive number to be in the range 1.5 − 2.0 (interval at the 1/10 relative likelihood). We simulated the international spreading of the outbreak and provide the estimate for the probability of Ebola virus disease case importation in countries across the world. Results indicate that the short-term (3 and 6 weeks) probability of international spread outside the African region is small, but not negligible. The extension of the outbreak is more likely occurring in African countries, increasing the risk of international dissemination on a longer time scale.

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worked on a model to estimate the growth of the disease,  

So, the longer the Ebola Virus has hosts the more likely it will mutate and be airborne eventually. It is said some strains are already carried by coughing to others already through the air.