Okay, I have a theory about this. Using information obtained by following the work of Al Gore, I've figured out the problem.
See, Manbearpig drowned, right? But what happened was the Republican party, acting on orders from Karl Rove and the Emperor, took samples of Manbearpig's DNA and used it in research to develop a virus that would affect both humans and pigs. (It will also affect bears, but at this point that's neither here nor there.) Now, we all know that the Republicans hate anyone who isn't white, so they decided to test this virus in Mexico. They infected a bunch of pigs, intending for the virus to be spread to the Mexican people via direct contact with the animals.
Unfortunately, because forethought isn't their strong suit, the Republicans didn't realize that the virus would actually become airborne and spread that way. Their little experiment got out of hand. By the time they caught on and were prepared to employ their emergency shutdown plan (Dick Cheney was going to go to Mexico and shoot all infected people and animals in the face), the infection had spread too far. Changing air currents caused by global warming had facilitated mass spread of this virus.
You guys, I'm super serial here. The Republican party, using Manbearpig's DNA, gave us all swine flu.