People in large-scale life-and-death scenarios do not akways behave rationally... and "stay put in one area with other affected people" is NOT a universal reaction to the problem.
Your right, I should have made myself more clear. People in healthy areas will not allow people from infected areas in. Quarantines work both ways.
In the days of the plague, ports would fire on plague ships trying to enter their harbors.
That was my point.