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The entire goal, is to cripple your target, consume his resources, demoralize and weaken your target. Mexico is a lousy target.

Going with a bioweapon theory and playing devil's advocate.

1) Bad health infrastructure. (unlike say ... Canada)
2) Cripple Mexico outright and where to they run to? USA
3) That either cripples the USA with health cases (PC wins) or starts a ground war along the southern USA as the US militarizes the border and shoots plague carriers (non-PC wins)
4) March - April ... tourist season and infected people catch it and return to countries of origin.
5) How many arabs and/or asians are actually going to vacation IN Mexico of all places??
6) Mexico is not exactly known for its wonderful intelligence and investigation capabilities. And it's territorial to a fault if the USA wants to backtrack a possible bioweapon. 7) Easier to infiltrate than USA (this is debateable)

Mexico would make the PERFECT springboard to infect the USA with a bioweapon.