The virus may have infected more then what it appears. The virus might just be "hidding" (I can't think of the medical term) and waiting for whatever reason to pop out and kill those infected. and then spread like wild fire... or is that not at all how it works?
This occurs with
Lysogenic phage - viruses that infect bacteria and wait for a key signal like DNA damage to burst out of infected cells and produce numerous copies of itself. I'm pretty sure influenza does not fall under this category.
Is it possible that it's mostly killing Mexicans because that they're Mexicans? Just as other ethnicities can be immune to certain diseases and weak against other.
Well, this is actually a fair question, although the answer is never as simple as you'd like. You can't bundle the people of an entire nation as having a genetic similarity, as the human genome is just as varied
within a subpopulation as it is
between subpopulations. True, there are a few major "exceptions," most notably the high proportion of Sub-Saharan Africans with a copy of the sickle-cell gene since it provides protection against malaria, but the idea that a group as large as a nation, ethnicity, or race would be genetically different enough to be affected by a disease in a totally different way is a little far-fetched.
Ideas like this are still thrown around, though, especially in the field of
Pharmacogenomics.
Bidil is a pretty famous drug that was actually marketed as a race-specific (for African Americans) drug. Later research showed it was pretty much bullshit and all other races could respond to it equally well.
So basically: You can have a different PROPORTION of a population expressing a certain gene, but def. not something that would cause 10% flu mortality rate in Mexicans vs 1% in rest of the world. My guess is the discrepancy is mostly due to quality of heath care + living conditions