It just is not realistically possible for countries to be isolationist in this current era. Indeed, the entire world is interconnected by the Internet and other technologies.
Consider this fact that shows how the world is becoming smaller as we group together even more closely: 3,000 years ago there were about 600,000 independent world communities; now there are fewer than 200 such communities.
And when a disease breaks out like Ebola in Africa, with our means of transportation, such a disease can show up and infect people in distant other places.