According to the Mexican government, the man that Obama talked with while in his trip to our neighbor died within a week of meeting the president, but he did not have the flu though he died of a pneunmonia like disease, that resembled the flu.
Because the president has not shown signs of flu himself, he has not been examined. Flu symptoms should have shown up within days of the contact if the president had been infected in any way by his trip south.
See LA Times "Obama has no flu symptoms after Mexico visit" (currently there are no registration requirements)
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