We're lucky that Ebola and some other vicious diseases such as Marburg haven't come here yet from Africa.
Of course there's a huge difference in the wealth and mobilty between the Africans usually infected, their travel frequency, and as far as I know the lack of an "African New Year", so that North American Africans do not go back "home" annually, like the Chinese often do, to visit their families, and to celebrate, therebye exporting and importing viruses both ways.