Not so easily prevented, not least because it can take up to 21 days for symptoms to appear after exposure and it's almost impossible to track where everyone has been.

http://www.independent.co.uk/travel/news-and-advice/simon-calder-ebola-virus-enhanced-screening-will...

The spread needs to be curtailed in the source countries, and quickly, before it gets completely out of control.  Half the medical staff where the disease rife are infected and they don't have protected suits etc.  

Avaaz.org is calling for medically trained people in the West to volunteer to help and many have already.  Good on them - brave people and I hope that they keep safe in their mission to help.