I haven't been following the deaths of people in Mexico (and in the U.S., primarily in the Southwest) closely, but Tyler Cowen says
Some people are puzzled as to how human, pig, and bird strains of the flu have mixed together, but if you have spent any time in rural Mexico the answer is obvious: these creatures all live together in close quarters.
What worries me, personally, is that many of their cousins
in their backyards here in the densely populated San Fernando Valley. I don't know how many keep pigs, but I wouldn't be surprised.