An e-mail from someone who sounds like she knows what she’s talking about:

Well, I am an epidemiologist… …of infectious diseases, no less. And what Obama (and Napolitano before him) said is utterly preposterous. Every infected patient is a potential index case–that is, the case from whom an outbreak spreads. The math is very simple: more index cases, more opportunities for outbreaks. Enough little outbreaks and, well, I suspect you get the picture. Think of it as a forest full of old trees and lots of underbrush (that would be all the people who have no immunity to the disease, we call them “susceptibles”). Add a lightening strike and maybe there will be a fire or maybe just a little smoke. Now toss in a thrown match and another lightening strike or two or three or ten or twelve or several hundred. Now calculate the probability of forest fire (for the math phobes: trust me, it goes up. A lot.) Each little flame in that dry brush is an index case.
The barn door analogy is just nonsense (on stilts!). 91 cases in a population of 300 million can be managed and isolated. No need to add to the number unnecessarily. It may not be necessary to place shoulder to shoulder border guards, but some common sense about looking hard at people entering is a decent start at prevention. Most people know what sick people look like (if they’re human they’ve been sick and likely looked in the mirror–it doesn’t take a medical degree). Most people, even government workers, know how to take someone’s temperature or certainly can be trained to use an automatic one. Even if it doesn’t stop everyone, if it stops some that’s a low cost, low effort intervention with a good chance at achieving the outcome of decreasing the number of potential index cases entering the country.