If it is like the 1918 flu, very strong immune systems would seem to be a problem; immune system overreaction is what has been hypothesized to have caused the deaths of so many young people back then.
In the U.S. so far, we haven't seen this type of response (that we know of).
Perhaps Mexico today is more like our country was in 1918...people were exposed to all kinds of dirt, garbage, water contamination when they were children, thus creating very strong immune systems.
We've seen an increase in asthma and allergies here, which has been explained by some as being due to our obsession with antibacterials and hyper-cleanliness. Perhaps the fact that our immune systems are not as strong is a good thing in this case?
1918 had another factor: war.
Three million draft call-ups, massive crowding in transportation, training facilities, troop ships, bond rallies, etc.
Plus the stress on resources, including medical that war brings, and in dividing the nations attention and forcing a cold decision: react to epidemic or make war.
Hmm. Wonder if for this very reason the US, Canada, and Japan will weather this flu rather well, but the rest of the world get decimated?