Contact Tracing works when you have one guy that you know has it -- and you fan out like wildfire to catch all the people exposed. It'll probably work for the one U.S. case.
It does not work when you have tens of thousands of people infected, like we do in other African countries. People who are dying at home. People who have been in contact with who-knows-whom.
So we're entering a new phase of Ebola: places where the disease is everywhere and we have no contact tracing, and places that can catch one or two cases as they appear. Nigera is currently in the latter camp. I hope they stay there.
This is why scientists think the U.S. and other more advanced countries will do better: we'll be able to stomp on new cases, track the contacts down, and catch the secondaries.
This will continue to work -- until it doesn't. That is, if the disease can be contained elsewhere, a low volume of cross-border transmission should be manageable. If, however, it continues to spread to unknown places, and grows in immeasurable ways? Then at some point the trickle of cases overloads any healthcare system.