From the scientist (Professor Peter Piot) who discovered Ebola:
With ever growing mobility of people and travel, it’s not unlikely that people during the incubation period of Ebola - which is between 2 and 21 days after infection - may go to another country.
Ebola outbreaks are always happening in a context of poverty, dysfunctional health services with poor infection control and hygiene practices. If someone with Ebola is admitted to such an environment anywhere in the world, that will give rise to outbreaks.
Link (another Times of India article dated Aug 1, 2014):
http://timesofindia.indiatimes.com/home/science/Its-the-deadliest-strain-says-the-man-who-discovered-Ebola/articleshow/39434060.cms