Ebola spreads in Africa because their society doesn't understand it and their infrastructure lacks the capabilities to contain it without assistance from developed nations.
Thomas Duncan, for instance, contracted Ebola from carrying a sick pregnant woman to a car. Bad idea.
You aren't going to see it spread in developed countries because we believe Ebola is a thing and not demons or some shit, and we take the appropriate precautions to prevent it from spreading.
Dallas dropped the ball initially by releasing Duncan due to a failure in the chain of custody of his patient history, but has since performed well, quarantining everyone he came in contact with.
Basically...