They screen everybody leaving those countries for fever to ensure that they at least aren't symptomatic at the time of the flight. You must have symptoms to spread the disease.
http://www.nytimes.com/2014/10/01/health/airline-passenger-with-ebola-is-under-treatment-in-dallas.html
"The man, who was visiting relatives in the United States, was not ill during the flight, health officials said at a news conference Tuesday evening. Indeed, he was screened before he boarded the flight and had no fever. Because Ebola is not contagious until symptoms develop, there is “zero chance” that the patient infected anyone else on the flight, said Dr. Thomas R. Frieden, director of the disease centers. Ebola is spread only by direct contact with body fluids from someone who is ill."
The thing I find most troubling was that he sought medical attention on 9/26, but was sent away and then finally placed in hospital isolation on 9/28. To the doctor(s) from 9/26, fvcking hello McFly! Liberia, fever, WTF? This kind of thing is what shakes my confidence in the controlling picture that the CDC like to paint, some doctors are indeed morons.