There is no barrier. It would be very easy for a person infected with Ebola to come to the USA (whether knowing they were afflicted or not).
Have you been to Africa? I have. Borders are not that secure. People cross back and forth between borders all the time and bribes are paid. Plus there are many non Africans working in Africa. Chinese, Filipino, Vietnamese, etc. These people could cross into a non affected country and then fly from there to any airport in the world including ours.
If the disease does not run its course soon in Africa it is coming here and there is no practical way to keep it out. Bringing those two back under super controlled conditions is not the situation we should be worried about.
People are still flying to and from the affected countries every day.
Yes, I know. Its inevitable.