>An (admittedly cursory) look at Vietnam suggests that their major cities are on the coast. Perhaps the jungle separating them from other nations was enough to

>An (admittedly cursory) look at Vietnam suggests that their major cities are on the coast. Perhaps the jungle separating them from other nations was enough to make them act more like an island? Won't stop a plane. Hanoi is very close to a major Chinese city. And the border near Sa Pa is infamous for having movement over the border. The much more simple explanation is that the initial cases were dealt with before it could spiral out of control.