I am not saying immunity, I am saying lack of a factor in America that would be present in Africa that exacerbates the severity of an illness. Perhaps another germ which by itself is harmless.
The factor that makes the big difference is called sanitary sewer and more to the point clean water delivered to nearly every house in the country. Most people who die in epidemics historically have died from dehydration. The symptoms of Ebola make that happen quicker, but being able to get clean water from inside the houses of anyone infected will make things much easier for anyone caring for someone stricken, and the sewer will carry away much of the infected waste of those with milder symptoms. In most of Africa they do not have running water outside of the major cities, and I suspect that for many an outhouse would be an improvement over current practice.