I realize that, but with the refusal to ban flights, the useless temperature scans at a few airports, this will become epidemic before long.
The temperature scans are not totally useless. It is true, they won't catch all infected individuals, but they will catch contagious ones, thus saving fellow passengers.
If Duncan had been contagious when he boarded in Monrovia, I expect the mess would be bigger than it is by now.