“The problem,” Mr. Obama said the other day by way of explaining his reluctance to impose a ban, “is that in all the discussions I’ve had thus far with experts in the field, experts in infectious disease, is that a travel ban is less effective than measures we are currently instituting, that involve screening passengers who are coming from West Africa.”

A nice thought, of the kind experts often have. Except it assumes that the process of screening and keeping track of potentially infected passengers once they are in the U.S. will be carried out effectively. It assumes, further, that the problem of Ebola in America is about Ebola alone.

It isn’t. It’s also about the panic that even a handful of cases can stir in the public, and the inevitable economic and political costs of managing that panic. Already we’re a country in an uproar on the basis of three confirmed cases of the disease. What if there were 30—or 300?