NPR has a great summary and write up of a new study published in PLOS Currents: Outbreaks that calculated the likelihood of Ebola cases coming to the United States and other countries based on virtual airline traffic. The study concluded that within 3-6 weeks the ‘probability of international spread outside the African region is small, but not negligible.’
… the authors of a new analysis say many countries — including the U.S. — should gear up to recognize, isolate and treat imported cases of Ebola.
The probability of seeing at least one imported case of Ebola in the U.S. is as high as 18 percent by late September…
These predictions are based on the flow of airline passengers from West Africa and the difficulty of preventing an infected passenger from boarding a flight.
Luckily, the study looked at the probability of one case of Ebola spreading to these countries, not a more widespread outbreak. The authors of the study predict that any Ebola cases would be limited to small clusters of 1-3 people. If US hospitals are prepared, imported Ebola cases should be quite easily contained using isolation and proper infection control (which is the best measure to stop any spread of the ebola virus).