This is where things get complicated. If the ban is to target all passengers who traveled through West Africa, “that would require a substantial amount of coordination with our friends and partners overseas,” says Vladeck. Countries already share information about air passengers with others—the question is whether the U.S. can convince other countries’ travel ministries to share enough to be able to piece together travelers’ previous stops.

And there remains the question of legality. If the ban is limited to non-citizens, its legality is fairly clear: it’s up to the U.S. to determine its own immigration policy, and it can keep foreign nationals out as it wishes, says Vladeck.

But once we’re talking about a travel ban on American citizens who may be in those areas, “the calculus changes rather dramatically, because courts have generally recognized a right on the part of U.S. citizens to travel,” he says. The question then becomes one of due process: the government would have to make sure the ban allows citizens to demonstrate that they’re not a risk to public health, for example.