Illegal immigrants have nothing to do with it, and neither does climate change. Not even George Bush has anything to do with it!
In my opinion, the outlawing of DDT has everything to do with it.
To be carrying it, just where do you think that the mosquito picks it up in the first place? Mosquitoes don't create the Denque; they can only pick it up from someone who's infected. While most of those folks with it are south of the border, many of them are crossing the border illegally without any health screening.