Well the sigmoid curve you are referring to would be for the case that patients who have contracted the virus and survived would be immune or die. In an SIR model (http://en.wikipedia.org/wiki/Compartmental_models_in_epidemi...) that takes death rate and population number into account you would then expect a disease to either reach a disease free equilibrium or a endemic equilibrium. What I was suggesting is to couple those equations on the country level by modelling travel patterns (flow of Suscebtible Recovered Infected across borders), the result would most likely be that western countries would be able to reach a disease free equilibrium, as they have better recovery rates and lower infection rates, whereas ebola would remain endemic in africa.