It's already here, and it isn't killing anyone. It is mild. Before you decide to close the borders you need to understand the economic concequences that would have. Why in the world should we close the borders over a flu that responds to current medication, especially when it is already here!?
And if this gets into a full blown swine flu pandemic, that WON'T affect the economy?

No, by that time, we'll be scratching our heads and saying, "Dang, I guess we should have sealed the border a while ago when it would have still done some good."
In reality, 1 out of every 3 Mexicans could come across the border with a hacking cough, boils, sweat running from their pores due to high fever and carrying a sign reading, "I am carry five different deadly diseases" and the US government wouldn't do a thing to stop them, save a token few for the sake of appearance.

Ironically, we are advising US citizens to avoid nonessential travel
TO Mexico, but
NOTHING has been done to slow the tide of travelers coming
FROM Mexico, the epicenter of swine flu.
If I sound jaded, I am. Living in a border state for the past 20 years and seeing the US government embrace an open border policy by virtue of the fact that they do almost nothing to enforce the border, has made me a bit cynical. We will NEVER offend the Mexican government, even at the expense of our own citizens. That's just the way it is. Our federal government really doesn't dwell on what is best for US citizens in this area and I do not expect that to stop now.
If Europe and other regions enact a travel ban on the US because of swine flu eventually (realizing that for all practical purposes, there is no border between the US and Mexico) I cannot blame them in the least. They will be acting in the best interests of their citizens, and that is understandable. Who could blame them?
I hope this fizzles out. But whether it turns out to be a minor annoyance or something on the level of the Spanish flu pandemic of 1918, the US will react (in regard to Mexico) exactly the same way. Politics, and not common sense, will rule. Should the border be shut down today? Heck, I can't say. But what I can say is that I do not trust the US to make a wise decision EVER on that point, given their past actions. They are simply incapable of it. The decision (We will never close the border, no matter what.) has already been made and the government will simply FIND the facts to
justify that decision. And that would be true whether we had a Democrat or a Republican in The White House.
That is where my concern lies.....Less with what this swine flu may turn out to be (because that's out of our control) and more with how my government may react to it (or fail to react) because of politics. I've lost all faith with them to look out for US citizens when it comes to dealing with Mexico. After 20+ years, you wise up.
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As for SARS, DH's company (a huge one) cancelled all travel to Asia and that was a stunner. Many other companies did the same. As someone mentioned, Toronto was hit very hard, as SARS was identified as being present there. The US got off easy. However, my friend's two small children had two of the first cases of SARS in the US. One of them nearly died and the family all flew in to say goodbye before she passed. By some miracle, she survived, because at the time, the doctors barely knew what was going on. Yes, Asia is only a plane ride away, but it's night and day compared to being next to Mexico, a country with whom we share a virtual non-border.