At least there’s a minimal degree of control over people who arrive legally, since they mostly have passports with visa stamps that show where they come from and where they have been. OTOH illegals swarming in across the southern border don’t even have ghat minimal documentation. An Ebola carrier coming in there will spread it far and wide before he is caught.
Have you also considered antibiotic resistant germs? Even if this ebola crisis doesn’t expand too much more. That’s still a serious concern.