I wrote a few days ago that closing the border with Mexico in response to the swine flu is not reasonable. Apparently even President Obama was asked the question last night in his press conference and he responded that "closing the door after the horse has left the barn" doesn't solve anything. He's right.

Since then I've heard more and more people asking that same question, including this afternoon on a talk show where there's both a conservative and a liberal host--and they both were asking why they aren't closing the border.

Since so many people are asking the same question I'm going to take another shot at explaining why it doesn't make any sense to close the border. I wish I could put together a graphical demonstration that would picture a map of the world and show a theoretical spread of the virus with and without closing the border, but I just don't have that kind of time.

What people don't seem to be remembering is that when someone has the virus, they don't just infect one person. They can infect everyone they come in contact with (though usually not everyone actually gets infected). This means the virus is spreading "exponentially." So you have a situation, for example, where:

1. One person in New York is sick and exposes the virus to 20 people.
   
2. Of those 20 people, perhaps 25% (5) get sick and expose it to another 20 people each. 5 sick people x 20 = 100 more people exposed.
   
3. Of those 100 people another 25% (25) get sick and expose it to another 20 people each. 25 x 20 = 500 more people exposed.
   
4. Of the 500 people, 25% (125) get sick... and so on.

So you have one case in New York and within a day or two you have 156 sick people. And that is just going to keep on spreading. And in such a mobile society you will see at least a few of those sick people fly to, say, Colorado--possibly before they even know they're sick. Then the whole process starts again in Colorado. And if they were sneezing on an airplane and exposing others on the plane, you might have those other people starting the process in other cities simultaneously.

The point is that if this virus actually spreads easily (which isn't really known yet) then the fact that we already have "launching points" throughout the country (12 states so far) means it will spread to every nook and cranny of the country. It's a given. Not everyone will get sick but the opportunity to get sick will exist nationwide. If this flu is highly contagious then it simply won't matter if you close the border. The fact that it's already in multiple places in the U.S. means it's guaranteed to spread nationwide.

The other possibility is that the swine flu is not highly contagious in which case it won't spread from New York... and in which case closing the border with Mexico would be an overreaction.

The only time it would make sense to close the border would be before there was a case of the virus in the country. If the threat were detected in Mexico before any infected person returned to the U.S. then it would make sense to close the border: But only if 1) every country in the world isolated all travel to/from Mexico or 2) we prohibited all international travel to/from everywhere.

That's because if we close the border with Mexico but travel is still permitted from Mexico to other countries, then eventually the virus would be brought indirectly from that other country to the U.S. There are already cases of the swine flu worldwide so what's the point of closing just one point of entry? If there were no cases in the U.S. it would still be pointless to close the border with Mexico if the virus could travel from Mexico to some other country and then to the U.S.

Example: Cuba has shut down all flights between Cuba and Mexico but flights to other international destinations are apparently still flying. If the swine virus is highly contagious then we're eventually going to see cases in Cuba despite their ban on flights from Mexico. And if the swine virus isn't highly contagious then the ban was pointless anyway.

Having said that, if you fear this swine flu then it does make sense to avoid crowds where there is an increased chance of exposure. In this case the flu will probably spread throughout the country (if it's highly contagious) but by avoiding large crowds you are decreasing your probability of exposure--so that, hopefully, by the time you go back to crowded situations the storm will have passed, so to speak.

But just because avoiding crowds makes sense doesn't mean that closing the border would help. Closing the border would just be a political act to make citizens feel safer even though it doesn't help... a lot like many of the tactics used by TSA at airports that are more for show than anything.

Update a few minutes later: An article at Time explains the same thing in more general terms, supported by models of biostatisticians.