Also, given the economic opportunities that the US, Russiw, and China have in Africa, just because the US shuts down flights would not mean other international travellers would do the same. In the interconnected world we live in, it is difficult and unlikely to stop all movement out of a continent. Once viruses reached other areas of the globe, there is no telling who they could infect, and where those travellers would end up.

Look at the Clade X exercises and see the recommendations and plans out in place. Despite those plans, it is nearly impossible to stop a virus that wants to spread. Sure, ban all flights from Africa to the US, but in a real pandemic, people might fly to countries far away from the epicenter, still be healthy during the incubation period and move onto the next stop.

Likely, for Ebola, it would need more mutations. Especially those that cause respiratory symptoms, this easily making the virus airborne and causing fomites.

For many other viruses, this already happens. Many of which need rather slight mutations to spread easily.

Bottom line is this, the more hosts a virus finds, the greater the likelihood of them adapting, causing easier modes of transmission. We have a few decades at most of understanding how these viruses work, much is still unknown.